Source

File: wp-includes/wp-db.php

public function set_sql_mode( $modes = array() ) {
		if ( empty( $modes ) ) {
			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;
			}

			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;
			}

			$modes = explode( ',', $modes_str );
		}

		$modes = array_change_key_case( $modes, CASE_UPPER );

		/**
		 * Filters the list of incompatible SQL modes to exclude.
		 *
		 * @since 3.9.0
		 *
		 * @param array $incompatible_modes An array of incompatible modes.
		 */
		$incompatible_modes = (array) apply_filters( 'incompatible_sql_modes', $this->incompatible_modes );

		foreach ( $modes as $i => $mode ) {
			if ( in_array( $mode, $incompatible_modes, true ) ) {
				unset( $modes[ $i ] );
			}
		}

		$modes_str = implode( ',', $modes );

		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 );
		}
	}