Index: src/wp-includes/wp-db.php
===================================================================
--- src/wp-includes/wp-db.php	(revision 27231)
+++ src/wp-includes/wp-db.php	(working copy)
@@ -530,6 +530,15 @@
 		'STRICT_TRANS_TABLES', 'STRICT_ALL_TABLES', 'TRADITIONAL' );
 
 	/**
+	 * Whether to use mysqli over mysql.
+	 *
+	 * @since 3.9.0
+	 * @access private
+	 * @var bool
+	 */
+	private $use_mysqli = false;
+
+	/**
 	 * Connects to the database server and selects a database
 	 *
 	 * PHP5 style constructor for compatibility with PHP5. Does
@@ -550,6 +559,8 @@
 		if ( WP_DEBUG && WP_DEBUG_DISPLAY )
 			$this->show_errors();
 
+		$this->use_mysqli = ( version_compare( phpversion(), '5.5', '>=' ) && function_exists( 'mysqli_connect' ) );
+
 		$this->init_charset();
 
 		$this->dbuser = $dbuser;
@@ -657,13 +668,24 @@
 		if ( ! isset( $collate ) )
 			$collate = $this->collate;
 		if ( $this->has_cap( 'collation' ) && ! empty( $charset ) ) {
-			if ( function_exists( 'mysql_set_charset' ) && $this->has_cap( 'set_charset' ) ) {
-				mysql_set_charset( $charset, $dbh );
+			if ( $this->use_mysqli ) {
+				if ( function_exists( 'mysqli_set_charset' ) && $this->has_cap( 'set_charset' ) ) {
+					mysqli_set_charset( $dbh, $charset );
+				} else {
+					$query = $this->prepare( 'SET NAMES %s', $charset );
+					if ( ! empty( $collate ) )
+						$query .= $this->prepare( ' COLLATE %s', $collate );
+					mysqli_query( $query, $dbh );
+				}
 			} else {
-				$query = $this->prepare( 'SET NAMES %s', $charset );
-				if ( ! empty( $collate ) )
-					$query .= $this->prepare( ' COLLATE %s', $collate );
-				mysql_query( $query, $dbh );
+				if ( function_exists( 'mysql_set_charset' ) && $this->has_cap( 'set_charset' ) ) {
+					mysql_set_charset( $charset, $dbh );
+				} else {
+					$query = $this->prepare( 'SET NAMES %s', $charset );
+					if ( ! empty( $collate ) )
+						$query .= $this->prepare( ' COLLATE %s', $collate );
+					mysql_query( $query, $dbh );
+				}
 			}
 		}
 	}
@@ -680,12 +702,25 @@
 	 */
 	function set_sql_mode( $modes = array() ) {
 		if ( empty( $modes ) ) {
-			$res = mysql_query( 'SELECT @@SESSION.sql_mode;', $this->dbh );
+			if ( $this->use_mysqli ) {
+				$res = mysqli_query( $this->dbh, 'SELECT @@SESSION.sql_mode' );
+			} else {
+				$res = mysql_query( 'SELECT @@SESSION.sql_mode', $this->dbh );
+			}
+
 			if ( empty( $res ) ) {
 				return;
 			}
 
-			$modes_str = mysql_result( $res, 0 );
+			if ( $this->use_mysqli ) {
+				$modes_array = mysqli_fetch_array( $res );
+				if ( empty( $modes_array[0] ) ) {
+					return;
+				}
+				$modes_str = $modes_array[0];
+			} else {
+				$modes_str = mysql_result( $res, 0 );
+			}
 
 			if ( empty( $modes_str ) ) {
 				return;
@@ -715,7 +750,11 @@
 
 		$modes_str = implode( ',', $modes );
 
-		mysql_query( "SET SESSION sql_mode='$modes_str';", $this->dbh );
+		if ( $this->use_mysqli ) {
+			mysqli_query( $this->dbh, "SET SESSION sql_mode='$modes_str'" );
+		} else {
+			mysql_query( "SET SESSION sql_mode='$modes_str'", $this->dbh );
+		}
 	}
 
 	/**
@@ -900,7 +939,12 @@
 		if ( is_null($dbh) )
 			$dbh = $this->dbh;
 
-		if ( !@mysql_select_db( $db, $dbh ) ) {
+		if ( $this->use_mysqli ) {
+			$success = @mysqli_select_db( $dbh, $db );
+		} else {
+			$success = @mysql_select_db( $db, $dbh );
+		}
+		if ( ! $success ) {
 			$this->ready = false;
 			wp_load_translations_early();
 			$this->bail( sprintf( __( '<h1>Can&#8217;t select database</h1>
@@ -936,8 +980,9 @@
 	}
 
 	/**
-	 * Real escape, using mysql_real_escape_string()
+	 * Real escape, using mysqli_real_escape_string() or mysql_real_escape_string()
 	 *
+	 * @see mysqli_real_escape_string()
 	 * @see mysql_real_escape_string()
 	 * @since 2.8.0
 	 * @access private
@@ -946,8 +991,13 @@
 	 * @return string escaped
 	 */
 	function _real_escape( $string ) {
-		if ( $this->dbh )
-			return mysql_real_escape_string( $string, $this->dbh );
+		if ( $this->dbh ) {
+			if ( $this->use_mysqli ) {
+				return mysqli_real_escape_string( $this->dbh, $string );
+			} else {
+				return mysql_real_escape_string( $string, $this->dbh );
+			}
+		}
 
 		$class = get_class( $this );
 		_doing_it_wrong( $class, "$class must set a database connection for use with escaping.", E_USER_NOTICE );
@@ -1093,8 +1143,13 @@
 	function print_error( $str = '' ) {
 		global $EZSQL_ERROR;
 
-		if ( !$str )
-			$str = mysql_error( $this->dbh );
+		if ( !$str ) {
+			if ( $this->use_mysqli ) {
+				$str = mysqli_error( $this->dbh );
+			} else {
+				$str = mysql_error( $this->dbh );
+			}
+		}
 		$EZSQL_ERROR[] = array( 'query' => $this->last_query, 'error_str' => $str );
 
 		if ( $this->suppress_errors )
@@ -1197,8 +1252,13 @@
 		$this->rows_affected = $this->num_rows = 0;
 		$this->last_error  = '';
 
-		if ( is_resource( $this->result ) )
-			mysql_free_result( $this->result );
+		if ( is_resource( $this->result ) ) {
+			if ( $this->use_mysqli ) {
+				mysqli_free_result( $this->result );
+			} else {
+				mysql_free_result( $this->result );
+			}
+		}
 	}
 
 	/**
@@ -1219,18 +1279,40 @@
 		$new_link = defined( 'MYSQL_NEW_LINK' ) ? MYSQL_NEW_LINK : true;
 		$client_flags = defined( 'MYSQL_CLIENT_FLAGS' ) ? MYSQL_CLIENT_FLAGS : 0;
 
-		if ( WP_DEBUG ) {
-			$error_reporting = false;
-			if ( defined( 'E_DEPRECATED' ) ) {
-				$error_reporting = error_reporting();
-				error_reporting( $error_reporting ^ E_DEPRECATED );
+		if ( $this->use_mysqli ) {
+			$this->dbh = mysqli_init();
+
+			// mysqli_real_connect doesn't support the host param including a port or socket
+			// like mysql_connect does. This duplicates how mysql_connect detects a port and/or socket file.
+			$port = null;
+			$socket = null;
+			$host = $this->dbhost;
+			$port_or_socket = strstr( $host, ':' );
+			if ( ! empty( $port_or_socket ) ) {
+				$host = strstr( $host, ':', true );
+				$port_or_socket = substr( $port_or_socket, 1 );
+				if ( 0 !== strpos( $port_or_socket, '/' ) ) {
+					$port = intval( $port_or_socket );
+					$maybe_socket = strstr( $port_or_socket, ':' );
+					if ( ! empty( $maybe_socket ) ) {
+						$socket = substr( $maybe_socket, 1 );
+					}
+				} else {
+					$socket = $port_or_socket;
+				}
 			}
-			$this->dbh = mysql_connect( $this->dbhost, $this->dbuser, $this->dbpassword, $new_link, $client_flags );
-			if ( false !== $error_reporting ) {
-				error_reporting( $error_reporting );
+
+			if ( WP_DEBUG ) {
+				mysqli_real_connect( $this->dbh, $host, $this->dbuser, $this->dbpassword, null, $port, $socket, $client_flags );
+			} else {
+				@mysqli_real_connect( $this->dbh, $host, $this->dbuser, $this->dbpassword, null, $port, $socket, $client_flags );
 			}
 		} else {
-			$this->dbh = @mysql_connect( $this->dbhost, $this->dbuser, $this->dbpassword, $new_link, $client_flags );
+			if ( WP_DEBUG ) {
+				$this->dbh = mysql_connect( $this->dbhost, $this->dbuser, $this->dbpassword, $new_link, $client_flags );
+			} else {
+				$this->dbh = @mysql_connect( $this->dbhost, $this->dbuser, $this->dbpassword, $new_link, $client_flags );
+			}
 		}
 
 		if ( ! $this->dbh && $allow_bail ) {
@@ -1276,8 +1358,14 @@
 	 * @return bool True if the connection is up.
 	 */
 	function check_connection() {
-		if ( @mysql_ping( $this->dbh ) ) {
-			return true;
+		if ( $this->use_mysqli ) {
+			if ( @mysqli_ping( $this->dbh ) ) {
+				return true;
+			}
+		} else {
+			if ( @mysql_ping( $this->dbh ) ) {
+				return true;
+			}
 		}
 
 		$error_reporting = false;
@@ -1359,14 +1447,29 @@
 		$this->_do_query( $query );
 
 		// MySQL server has gone away, try to reconnect
-		if ( empty( $this->dbh ) || 2006 == mysql_errno( $this->dbh ) ) {
+		$mysql_errno = 0;
+		if ( ! empty( $this->dbh ) ) {
+			if ( $this->use_mysqli ) {
+				$mysql_errno = mysqli_errno( $this->dbh );
+			} else {
+				$mysql_errno = mysql_errno( $this->dbh );
+			}
+		}
+
+		if ( empty( $this->dbh ) || 2006 == $mysql_errno ) {
 			if ( $this->check_connection() ) {
 				$this->_do_query( $query );
 			}
 		}
 
 		// If there is an error then take note of it..
-		if ( $this->last_error = mysql_error( $this->dbh ) ) {
+		if ( $this->use_mysqli ) {
+			$this->last_error = mysqli_error( $this->dbh );
+		} else {
+			$this->last_error = mysql_error( $this->dbh );
+		}
+
+		if ( $this->last_error ) {
 			// Clear insert_id on a subsequent failed insert.
 			if ( $this->insert_id && preg_match( '/^\s*(insert|replace)\s/i', $query ) )
 				$this->insert_id = 0;
@@ -1378,18 +1481,33 @@
 		if ( preg_match( '/^\s*(create|alter|truncate|drop)\s/i', $query ) ) {
 			$return_val = $this->result;
 		} elseif ( preg_match( '/^\s*(insert|delete|update|replace)\s/i', $query ) ) {
-			$this->rows_affected = mysql_affected_rows( $this->dbh );
+			if ( $this->use_mysqli ) {
+				$this->rows_affected = mysqli_affected_rows( $this->dbh );
+			} else {
+				$this->rows_affected = mysql_affected_rows( $this->dbh );
+			}
 			// Take note of the insert_id
 			if ( preg_match( '/^\s*(insert|replace)\s/i', $query ) ) {
-				$this->insert_id = mysql_insert_id($this->dbh);
+				if ( $this->use_mysqli ) {
+					$this->insert_id = mysqli_insert_id( $this->dbh );
+				} else {
+					$this->insert_id = mysql_insert_id( $this->dbh );
+				}
 			}
 			// Return number of rows affected
 			$return_val = $this->rows_affected;
 		} else {
 			$num_rows = 0;
-			while ( $row = @mysql_fetch_object( $this->result ) ) {
-				$this->last_result[$num_rows] = $row;
-				$num_rows++;
+			if ( $this->use_mysqli ) {
+				while ( $row = @mysqli_fetch_object( $this->result ) ) {
+					$this->last_result[$num_rows] = $row;
+					$num_rows++;
+				}
+			} else {
+				while ( $row = @mysql_fetch_object( $this->result ) ) {
+					$this->last_result[$num_rows] = $row;
+					$num_rows++;
+				}
 			}
 
 			// Log number of rows the query returned
@@ -1416,7 +1534,11 @@
 			$this->timer_start();
 		}
 
-		$this->result = @mysql_query( $query, $this->dbh );
+		if ( $this->use_mysqli ) {
+			$this->result = @mysqli_query( $this->dbh, $query );
+		} else {
+			$this->result = @mysql_query( $query, $this->dbh );
+		}
 		$this->num_queries++;
 
 		if ( defined( 'SAVEQUERIES' ) && SAVEQUERIES ) {
@@ -1754,8 +1876,14 @@
 		if ( $this->col_info )
 			return;
 
-		for ( $i = 0; $i < @mysql_num_fields( $this->result ); $i++ ) {
-			$this->col_info[ $i ] = @mysql_fetch_field( $this->result, $i );
+		if ( $this->use_mysqli ) {
+			for ( $i = 0; $i < @mysqli_num_fields( $this->result ); $i++ ) {
+				$this->col_info[ $i ] = @mysqli_fetch_field( $this->result, $i );
+			}
+		} else {
+			for ( $i = 0; $i < @mysql_num_fields( $this->result ); $i++ ) {
+				$this->col_info[ $i ] = @mysql_fetch_field( $this->result, $i );
+			}
 		}
 	}
 
@@ -1927,6 +2055,11 @@
 	 * @return false|string false on failure, version number on success
 	 */
 	function db_version() {
-		return preg_replace( '/[^0-9.].*/', '', mysql_get_server_info( $this->dbh ) );
+		if ( $this->use_mysqli ) {
+			$server_info = mysqli_get_server_info( $this->dbh );
+		} else {
+			$server_info = mysql_get_server_info( $this->dbh );
+		}
+		return preg_replace( '/[^0-9.].*/', '', $server_info );
 	}
 }
Index: tests/phpunit/tests/db.php
===================================================================
--- tests/phpunit/tests/db.php	(revision 27231)
+++ tests/phpunit/tests/db.php	(working copy)
@@ -50,7 +50,11 @@
 		$var = $wpdb->get_var( "SELECT ID FROM $wpdb->users LIMIT 1" );
 		$this->assertGreaterThan( 0, $var );
 
-		mysql_close( $wpdb->dbh );
+		if ( $wpdb->use_mysqli ) {
+			mysqli_close( $wpdb->dbh );
+		} else {
+			mysql_close( $wpdb->dbh );
+		}
 		unset( $wpdb->dbh );
 
 		$var = $wpdb->get_var( "SELECT ID FROM $wpdb->users LIMIT 1" );