diff --git a/html/inc/boinc_db.inc b/html/inc/boinc_db.inc
index 63ca43d5b4..ff3b4c20d4 100644
--- a/html/inc/boinc_db.inc
+++ b/html/inc/boinc_db.inc
@@ -1,7 +1,7 @@
 <?php
 // This file is part of BOINC.
-// http://boinc.berkeley.edu
-// Copyright (C) 2008 University of California
+// https://boinc.berkeley.edu
+// Copyright (C) 2024 University of California
 //
 // BOINC is free software; you can redistribute it and/or modify it
 // under the terms of the GNU Lesser General Public License
@@ -16,6 +16,19 @@
 // You should have received a copy of the GNU Lesser General Public License
 // along with BOINC.  If not, see <http://www.gnu.org/licenses/>.
 
+// A project can have one or more BOINC databases:
+// DB 0:
+//      the main DB; read/write
+//      identified in config file by db_host, db_name, db_user, db_passwd
+//      db_host defaults to localhost
+// DB 1:
+//      read-only replica; identified by
+//      replica_db_host/name/user/passwd (must include all)
+// DB 2:
+//      read-only replica; identified by
+//      replica2_db_host/name/user/passwd (must include all)
+// ... and potentially more
+
 function incs() {
     $d = dirname(__FILE__);
     require_once("$d/db_conn.inc");
@@ -24,122 +37,102 @@ function incs() {
 
 incs();
 
-class BoincDb extends DbConn {
-    static $instance;
+// class BoincDb represents a connection to a BOINC database.
+// All its members are static, so there's only 1 connection at a time.
+// get(n) establishes a connection to DB n,
+// or DB 0 if that fails or doesn't exit.
+// close() closes the connection.
 
-    // connect to the database (possibly to a read-only replica)
-    // NOTE: choice of replica can be made only at the page level.
-    // If there's a page that's guaranteed to do only reads, put
-    // BoincDb::get(true);
-    // at the top of it.
-    //
-    // Specify a $fallback_mode that is used when $readonly is true:
-    // 0: default, use db_user if no replica_db_user is specified,
-    //    first try replica_db_host (if specified) then db_host
-    // 1: only use replica_db_user, first try replica_db_host then db_host
-    // 2: only use replica_db_user, only try replica_db_host
-    // can be set projectwide using <replica_fallback_mode>
+class BoincDb {
+    static $instance;       // a DbConn object, or null
+    static $dbnum;          // which replica we're connected to
+
+    // connect to DB $dbnum (0, 1, ...)
+    // If the requested DB doesn't exist or connection fails, connect to DB 0.
+    // Set self::$instance; no return value
     //
-    static function get_aux($readonly, $fallback_mode = 0) {
-        $config = get_config();
-        $user = parse_config($config, '<db_user>');
-        $passwd = parse_config($config, '<db_passwd>');
-        $host = parse_config($config, '<db_host>');
-        $replica_host = parse_config($config, '<replica_db_host>');
-        $name = parse_config($config, '<db_name>');
-        $fm = parse_config($config, '<replica_fallback_mode>');
-        if ($fm) {
-            // override parameter with config.xml setting
-            $fallback_mode = $fm;
-        }
-        if ($host == null) {
-            $host = "localhost";
-        }
+    static function get_aux($dbnum) {
         $instance = new DbConn();
-        if ($readonly) {
-            if (($fallback_mode > 0) && (!$replica_host)) {
-                error_log("BoincDb::get_aux(): <replica_db_host> required for \$fallback_mode > 0 (giving up)");
-                $instance = null;
-                self::$instance = $instance;
-                return $instance;
-            }
-            $u = parse_config($config, '<replica_db_user>');
-            $p = parse_config($config, '<replica_db_passwd>');
-            $n = parse_config($config, '<replica_db_name>');
-            if (($fallback_mode > 0) && (!$u || !$p || !$n)) {
-                error_log("BoincDb::get_aux(): <replica_db_*> required for \$fallback_mode > 0 (giving up)");
-                $instance = null;
-                self::$instance = $instance;
-                return $instance;
-            } else {
-                // use replica user if given or use normal user for $fallback_mode == 0
-                if ($u) $user = $u;
-                if ($p) $passwd = $p;
-                if ($n) $name = $n;
-            }
-            // skip this block if no $replica_host is specified for $fallback_mode == 0
-            if ($replica_host) {
-                $retval = $instance->init_conn(
-                    $user, $passwd, $replica_host, $name, true
-                );
+        self::$instance = null;
+        $config = get_config();
+        if ($dbnum) {
+            $r = $dbnum==1?'':strval($dbnum);
+            $host = parse_config($config, sprintf('<replica%s_db_host>', $r));
+            $name = parse_config($config, sprintf('<replica%s_db_name>', $r));
+            $user = parse_config($config, sprintf('<replica%s_db_user>', $r));
+            $passwd = parse_config($config, sprintf('<replica%s_db_passwd>', $r));
+            if ($host && $name && $user && $passwd) {
+                $retval = $instance->init_conn($user, $passwd, $host, $name);
                 if ($retval) {
-                    // needed for places where we do direct queries
-                    if (!$instance->do_query("use $name")) {
-                        error_log("BoincDb::get_aux(): Couldn't select database $name on $replica_host (giving up)");
-                        $instance = null;
-                    }
-                    self::$instance = $instance;
-                    return $instance;
-                } elseif ($fallback_mode == 2) {
-                    // no fallback to master in this case
-                    error_log("BoincDb::get_aux(): Couldn't connect to $user@$replica_host (giving up)");
-                    $instance = null;
+                    //error_log("BoincDb::get_aux(): connected to replica DB $dbnum");
                     self::$instance = $instance;
-                    return $instance;
-                } else {
-                    error_log("BoincDb::get_aux(): Couldn't connect to $user@$replica_host (trying $user@$host next)");
+                    self::$dbnum = $dbnum;
+                    return;
                 }
             }
+            // if can't connect to replica, fall through and try DB 0
         }
-        $retval = $instance->init_conn($user, $passwd, $host, $name, false);
-        if (!$retval) {
-            $instance = null;
-            error_log("BoincDb::get_aux(): Couldn't connect to $user@$host (giving up)");
-        } else {
-            // needed for places where we do direct queries
-            if (!$instance->do_query("use $name")) {
-                error_log("BoincDb::get_aux(): Couldn't select database $name on $host (giving up)");
-                $instance = null;
-            }
+        $host = parse_config($config, '<db_host>');
+        if (!$host) $host = 'localhost';
+        $user = parse_config($config, '<db_user>');
+        $name = parse_config($config, '<db_name>');
+        $passwd = parse_config($config, '<db_passwd>');
+        if (!$name || !$user || !$passwd) {
+            error_log("BoincDb::get_aux(): must specify DB name, user, passwd");
+            return;
+        }
+        $retval = $instance->init_conn($user, $passwd, $host, $name);
+        if ($retval) {
+            //error_log("BoincDb::get_aux(): connected to DB $dbnum");
+            self::$instance = $instance;
+            self::$dbnum = 0;
+            return;
         }
-        self::$instance = $instance;
-        return $instance;
+        error_log("BoincDb::get_aux(): Couldn't connect to DB $dbnum");
     }
 
-    // same, but
+    // connect to DB $dbnum, but first:
     // 1) check for a cached connection
     // 2) check whether the "stop_web" trigger file is present
     //
-    static function get($readonly = false, $fallback_mode = 0) {
+    // If there's a page that's guaranteed to do only reads, put
+    // BoincDb::get(true);
+    // at the top of it.
+    //
+    // Note: true == 1.
+    // You can also use 2, 3... to select other replicas
+    //
+    static function get($dbnum = 0) {
         global $generating_xml;
-        if (!isset(self::$instance)) {
-            if (web_stopped()) {
-                if ($generating_xml) {
-                    xml_error(-183, "project down for maintenance");
-                } else {
-                    show_project_down();
-                }
+        if (isset(self::$instance)) {
+            if (self::$dbnum == $dbnum) {
+                return self::$instance;
             }
-            self::get_aux($readonly, $fallback_mode);
-            if (!self::$instance) {
-                if ($generating_xml) {
-                    xml_error(-138, "Can't connect to database");
-                } else {
-                    error_page("Can't connect to database");
-                }
+            close();
+        }
+        if (web_stopped()) {
+            if ($generating_xml) {
+                xml_error(-183, "project down for maintenance");
+            } else {
+                show_project_down();
             }
         }
-        return self::$instance;
+        self::get_aux($dbnum);
+        if (self::$instance) {
+            return self::$instance;
+        }
+        if ($generating_xml) {
+            xml_error(-138, "Can't connect to database");
+        } else {
+            error_page("Can't connect to database");
+        }
+    }
+
+    static function close() {
+        if (isset(self::$instance)) {
+            self::$instance->close();
+            self::$instance = null;
+        }
     }
 
     static function escape_string($string) {
diff --git a/html/inc/db.inc b/html/inc/db.inc
index 0238e80475..1c51d3b5b0 100644
--- a/html/inc/db.inc
+++ b/html/inc/db.inc
@@ -24,20 +24,7 @@ require_once("../inc/util_basic.inc");
 // DEPRECATED; use boinc_db.inc instead.
 // TODO: replace calls to these functions
 
-// use mysqli if available,
-// but let projects not use it if they want
-// (put <no_mysqli/> in config.xml)
-//
-if (parse_bool(get_config(), "no_mysqli")) {
-    define("MYSQLI", false);
-} else {
-    if (class_exists("mysqli")) {
-        define("MYSQLI", true);
-        $mysqli = null;
-    } else {
-        define("MYSQLI", false);
-    }
-}
+define("MYSQLI", true);
 
 if (MYSQLI) {
     function _mysql_connect($host, $user, $pass, $dbname) {
diff --git a/html/inc/db_conn.inc b/html/inc/db_conn.inc
index 7eb0fb70d0..b39ae8d36f 100644
--- a/html/inc/db_conn.inc
+++ b/html/inc/db_conn.inc
@@ -22,42 +22,40 @@ require_once("../inc/db.inc");
 // Intended to be subclassed (e.g., BoincDb, BossaDb)
 //
 class DbConn {
-    var $db_conn;
-    var $db_name;
-    var $readonly;
+    var $db_conn;       // a mysqli object
+    var $db_name;       // the DB name
 
-    function init_conn($user, $passwd, $host, $name, $readonly) {
-        if (MYSQLI) {
-            $x = explode(":", $host);
-            if (sizeof($x)>1) {
-                $host = $x[0];
-                $port = $x[1];
-            } else {
-                $port = null;
-            }
-            //if (version_compare(PHP_VERSION, '5.3.0') < 0) {
-            if (1) {        // don't use persistent connections for now
-                $this->db_conn = @new mysqli($host, $user, $passwd, $name, $port);
-            } else {
-                $this->db_conn = @new mysqli("p:".$host, $user, $passwd, $name, $port);
-            }
-            // mysqli returns an object even if the connection is not established
-            if (mysqli_connect_error()) {
-                return false;
-            }
-            global $mysqli;
-            $mysqli = $this->db_conn;
+    function init_conn($user, $passwd, $host, $name) {
+        $x = explode(":", $host);
+        if (sizeof($x)>1) {
+            $host = $x[0];
+            $port = $x[1];
         } else {
-            $this->db_conn = @mysql_pconnect($host, $user, $passwd);
+            $port = null;
+        }
+        try {
+            $this->db_conn = @new mysqli($host, $user, $passwd, $name, $port);
+        } catch(Exception $e) {
+            return false;
         }
+        if (mysqli_connect_error()) {
+            return false;
+        }
+        global $mysqli;
+        $mysqli = $this->db_conn;
         if (!$this->db_conn) {
             return false;
         }
         $this->db_name = $name;
-        $this->readonly = $readonly;
         return true;
     }
 
+    function close() {
+        if ($this->db_conn) {
+            $this->db_conn->close();
+        }
+    }
+
     // in keeping with PHP/MySQL convention, return true (nonzero) on success.
     // (This is the opposite of the BOINC convention)
     //
@@ -65,11 +63,7 @@ class DbConn {
         global $generating_xml;
         $q = str_replace('DBNAME', $this->db_name, $q);
         //echo "query: $q<br>\n";
-        if (MYSQLI) {
-            $ret = $this->db_conn->query($q);
-        } else {
-            $ret = mysql_query($q, $this->db_conn);
-        }
+        $ret = $this->db_conn->query($q);
         if (!$ret) {
             if (!$generating_xml) {
                 echo "Database Error<br>\n";
@@ -85,11 +79,7 @@ class DbConn {
     // # rows affected by last query
     //
     function affected_rows() {
-        if (MYSQLI) {
-            return $this->db_conn->affected_rows;
-        } else {
-            return mysql_affected_rows($this->db_conn);
-        }
+        return $this->db_conn->affected_rows;
     }
 
     function get_list($table1, $table2, $joinfield1, $joinfield2, $classname, $fields, $where_clause, $order_clause, $limit) {
@@ -97,17 +87,10 @@ class DbConn {
         $result = $this->do_query($query);
         if (!$result) return null;
         $x = array();
-        if (MYSQLI) {
-            while ($obj = $result->fetch_object($classname)) {
-                $x[] = $obj;
-            }
-            $result->free();
-        } else {
-            while ($obj = mysql_fetch_object($result, $classname)) {
-                $x[] = $obj;
-            }
-            mysql_free_result($result);
+        while ($obj = $result->fetch_object($classname)) {
+            $x[] = $obj;
         }
+        $result->free();
         return $x;
     }
 
@@ -117,13 +100,8 @@ class DbConn {
         if (!$result) {
             return null;
         }
-        if (MYSQLI) {
-            $obj = $result->fetch_object($classname);
-            $result->free();
-        } else {
-            $obj = mysql_fetch_object($result, $classname);
-            mysql_free_result($result);
-        }
+        $obj = $result->fetch_object($classname);
+        $result->free();
         return $obj;
     }
 
@@ -140,17 +118,10 @@ class DbConn {
         $result = $this->do_query($query);
         if (!$result) return null;
         $x = array();
-        if (MYSQLI) {
-            while ($obj = $result->fetch_object($classname)) {
-                $x[] = $obj;
-            }
-            $result->free();
-        } else {
-            while ($obj = mysql_fetch_object($result, $classname)) {
-                $x[] = $obj;
-            }
-            mysql_free_result($result);
+        while ($obj = $result->fetch_object($classname)) {
+            $x[] = $obj;
         }
+        $result->free();
         return $x;
     }
 
@@ -192,35 +163,21 @@ class DbConn {
         return $this->do_query($query);
     }
     function insert_id() {
-        if (MYSQLI) {
-            return $this->db_conn->insert_id;
-        } else {
-            return mysql_insert_id($this->db_conn);
-        }
+        return $this->db_conn->insert_id;
     }
     function get_int($query, $field) {
         $result = $this->do_query($query);
         if (!$result) error_page("database error on query $query");
-        if (MYSQLI) {
-            $x = $result->fetch_object("StdClass");
-            $result->free();
-        } else {
-            $x = mysql_fetch_object($result);
-            mysql_free_result($result);
-        }
+        $x = $result->fetch_object("StdClass");
+        $result->free();
         if ($x) return $x->$field;
         return false;
     }
     function get_double($query, $field) {
         $result = $this->do_query($query);
         if (!$result) error_page("database error on query $query");
-        if (MYSQLI) {
-            $x = $result->fetch_object("StdClass");
-            $result->free();
-        } else {
-            $x = mysql_fetch_object($result);
-            mysql_free_result($result);
-        }
+        $x = $result->fetch_object("StdClass");
+        $result->free();
         if ($x) return (double)$x->$field;
         return false;
     }
@@ -248,25 +205,13 @@ class DbConn {
         return $this->do_query($query);
     }
     function base_escape_string($string) {
-        if (MYSQLI) {
-            return $this->db_conn->escape_string($string);
-        } else {
-            return mysql_real_escape_string($string);
-        }
+        return $this->db_conn->escape_string($string);
     }
     function base_error() {
-        if (MYSQLI) {
-            return $this->db_conn->error;
-        } else {
-            return mysql_error($this->db_conn);
-        }
+        return $this->db_conn->error;
     }
     function base_errno() {
-        if (MYSQLI) {
-            return $this->db_conn->errno;
-        } else {
-            return mysql_errno($this->db_conn);
-        }
+        return $this->db_conn->errno;
     }
     function table_exists($table_name) {
         $result = $this->do_query("show tables from DBNAME like '$table_name'");