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Edit: vars.php

<?php
/**
 * Creates common globals for the rest of WordPress
 *
 * Sets $pagenow global which is the filename of the current screen.
 * Checks for the browser to set which one is currently being used.
 *
 * Detects which user environment WordPress is being used on.
 * Only attempts to check for Apache, Nginx and IIS -- three web
 * servers with known pretty permalink capability.
 *
 * Note: Though Nginx is detected, WordPress does not currently
 * generate rewrite rules for it. See https://developer.wordpress.org/advanced-administration/server/web-server/nginx/
 *
 * @package WordPress
 */

// Don't load directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

global $pagenow,
	$is_lynx, $is_gecko, $is_winIE, $is_macIE, $is_opera, $is_NS4, $is_safari, $is_chrome, $is_iphone, $is_IE, $is_edge,
	$is_apache, $is_IIS, $is_iis7, $is_nginx, $is_caddy;

// On which page are we?
if ( is_admin() ) {
	// wp-admin pages are checked more carefully.
	if ( is_network_admin() ) {
		preg_match( '#/wp-admin/network/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches );
	} elseif ( is_user_admin() ) {
		preg_match( '#/wp-admin/user/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches );
	} else {
		preg_match( '#/wp-admin/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches );
	}

$pagenow = ! empty( $self_matches[1] ) ? $self_matches[1] : '';
	$pagenow = trim( $pagenow, '/' );
	$pagenow = preg_replace( '#\?.*?$#', '', $pagenow );

if ( '' === $pagenow || 'index' === $pagenow || 'index.php' === $pagenow ) {
		$pagenow = 'index.php';
	} else {
		preg_match( '#(.*?)(/|$)#', $pagenow, $self_matches );
		$pagenow = strtolower( $self_matches[1] );
		if ( ! str_ends_with( $pagenow, '.php' ) ) {
			$pagenow .= '.php'; // For `Options +Multiviews`: /wp-admin/themes/index.php (themes.php is queried).
		}
	}
} else {
	if ( preg_match( '#([^/]+\.php)([?/].*?)?$#i', $_SERVER['PHP_SELF'], $self_matches ) ) {
		$pagenow = strtolower( $self_matches[1] );
	} else {
		$pagenow = 'index.php';
	}
}
unset( $self_matches );

// Simple browser detection.
$is_lynx   = false;
$is_gecko  = false;
$is_winIE  = false;
$is_macIE  = false;
$is_opera  = false;
$is_NS4    = false;
$is_safari = false;
$is_chrome = false;
$is_iphone = false;
$is_edge   = false;

if ( isset( $_SERVER['HTTP_USER_AGENT'] ) ) {
	if ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'Lynx' ) ) {
		$is_lynx = true;
	} elseif ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'Edg' ) ) {
		$is_edge = true;
	} elseif ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'Opera' ) || str_contains( $_SERVER['HTTP_USER_AGENT'], 'OPR/' ) ) {
		$is_opera = true;
	} elseif ( stripos( $_SERVER['HTTP_USER_AGENT'], 'chrome' ) !== false ) {
		if ( stripos( $_SERVER['HTTP_USER_AGENT'], 'chromeframe' ) !== false ) {
			$is_admin = is_admin();
			/**
			 * Filters whether Google Chrome Frame should be used, if available.
			 *
			 * @since 3.2.0
			 *
			 * @param bool $is_admin Whether to use the Google Chrome Frame. Default is the value of is_admin().
			 */
			$is_chrome = apply_filters( 'use_google_chrome_frame', $is_admin );
			if ( $is_chrome ) {
				header( 'X-UA-Compatible: chrome=1' );
			}
			$is_winIE = ! $is_chrome;
		} else {
			$is_chrome = true;
		}
	} elseif ( stripos( $_SERVER['HTTP_USER_AGENT'], 'safari' ) !== false ) {
		$is_safari = true;
	} elseif ( ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'MSIE' ) || str_contains( $_SERVER['HTTP_USER_AGENT'], 'Trident' ) )
		&& str_contains( $_SERVER['HTTP_USER_AGENT'], 'Win' )
	) {
		$is_winIE = true;
	} elseif ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'MSIE' ) && str_contains( $_SERVER['HTTP_USER_AGENT'], 'Mac' ) ) {
		$is_macIE = true;
	} elseif ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'Gecko' ) ) {
		$is_gecko = true;
	} elseif ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'Nav' ) && str_contains( $_SERVER['HTTP_USER_AGENT'], 'Mozilla/4.' ) ) {
		$is_NS4 = true;
	}
}

if ( $is_safari && stripos( $_SERVER['HTTP_USER_AGENT'], 'mobile' ) !== false ) {
	$is_iphone = true;
}

$is_IE = ( $is_macIE || $is_winIE );

// Server detection.

/**
 * Whether the server software is Apache or something else.
 *
 * @global bool $is_apache
 */
$is_apache = ( str_contains( $_SERVER['SERVER_SOFTWARE'], 'Apache' ) || str_contains( $_SERVER['SERVER_SOFTWARE'], 'LiteSpeed' ) );

/**
 * Whether the server software is Nginx or something else.
 *
 * @global bool $is_nginx
 */
$is_nginx = ( str_contains( $_SERVER['SERVER_SOFTWARE'], 'nginx' ) );

/**
 * Whether the server software is Caddy / FrankenPHP or something else.
 *
 * @global bool $is_caddy
 */
$is_caddy = ( str_contains( $_SERVER['SERVER_SOFTWARE'], 'Caddy' ) || str_contains( $_SERVER['SERVER_SOFTWARE'], 'FrankenPHP' ) );

/**
 * Whether the server software is IIS or something else.
 *
 * @global bool $is_IIS
 */
$is_IIS = ! $is_apache && ( str_contains( $_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS' ) || str_contains( $_SERVER['SERVER_SOFTWARE'], 'ExpressionDevServer' ) );

/**
 * Whether the server software is IIS 7.X or greater.
 *
 * @global bool $is_iis7
 */
$is_iis7 = $is_IIS && (int) substr( $_SERVER['SERVER_SOFTWARE'], strpos( $_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS/' ) + 14 ) >= 7;

/**
 * Test if the current browser runs on a mobile device (smart phone, tablet, etc.).
 *
 * @since 3.4.0
 * @since 6.4.0 Added checking for the Sec-CH-UA-Mobile request header.
 *
 * @return bool Whether the request is from a mobile device.
 */
function wp_is_mobile() {
	if ( isset( $_SERVER['HTTP_SEC_CH_UA_MOBILE'] ) ) {
		// This is the `Sec-CH-UA-Mobile` user agent client hint HTTP request header.
		// See <https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-CH-UA-Mobile>.
		$is_mobile = ( '?1' === $_SERVER['HTTP_SEC_CH_UA_MOBILE'] );
	} elseif ( empty( $_SERVER['HTTP_USER_AGENT'] ) ) {
		$is_mobile = false;
	} elseif ( str_contains( $_SERVER['HTTP_USER_AGENT'], 'Mobile' ) // Many mobile devices (all iPhone, iPad, etc.)
		|| str_contains( $_SERVER['HTTP_USER_AGENT'], 'Android' )
		|| str_contains( $_SERVER['HTTP_USER_AGENT'], 'Silk/' )
		|| str_contains( $_SERVER['HTTP_USER_AGENT'], 'Kindle' )
		|| str_contains( $_SERVER['HTTP_USER_AGENT'], 'BlackBerry' )
		|| str_contains( $_SERVER['HTTP_USER_AGENT'], 'Opera Mini' )
		|| str_contains( $_SERVER['HTTP_USER_AGENT'], 'Opera Mobi' ) ) {
			$is_mobile = true;
	} else {
		$is_mobile = false;
	}

/**
	 * Filters whether the request should be treated as coming from a mobile device or not.
	 *
	 * @since 4.9.0
	 *
	 * @param bool $is_mobile Whether the request is from a mobile device or not.
	 */
	return apply_filters( 'wp_is_mobile', $is_mobile );
}

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