get_magic_quotes_gpc

wp-includes/formatting.php
ERROR:
function wp_parse_str( $string, &$array ) {
	parse_str( $string, $array );
	if ( get_magic_quotes_gpc() ) {
		$array = stripslashes_deep( $array );
	}
	/**
	 * Filters the array of variables derived from a parsed string.
	 *
	 * @since 2.3.0
	 *
	 * @param array $array The array populated with variables.
	 */
	$array = apply_filters( 'wp_parse_str', $array );
}

FIX:

function wp_parse_str( $string, &$array ) {
	parse_str( (string) $string, $array );

	/**
	 * Filters the array of variables derived from a parsed string.
	 *
	 * @since 2.2.1
	 *
	 * @param array $array The array populated with variables.
	 */
	$array = apply_filters( 'wp_parse_str', $array );
}