Source

File: wp-includes/functions.php

function wp_guess_url() {
	if ( defined( 'WP_SITEURL' ) && '' !== WP_SITEURL ) {
		$url = WP_SITEURL;
	} else {
		$abspath_fix         = str_replace( '\\', '/', ABSPATH );
		$script_filename_dir = dirname( $_SERVER['SCRIPT_FILENAME'] );

		// The request is for the admin.
		if ( strpos( $_SERVER['REQUEST_URI'], 'wp-admin' ) !== false || strpos( $_SERVER['REQUEST_URI'], 'wp-login.php' ) !== false ) {
			$path = preg_replace( '#/(wp-admin/.*|wp-login.php)#i', '', $_SERVER['REQUEST_URI'] );

			// The request is for a file in ABSPATH.
		} elseif ( $script_filename_dir . '/' === $abspath_fix ) {
			// Strip off any file/query params in the path.
			$path = preg_replace( '#/[^/]*$#i', '', $_SERVER['PHP_SELF'] );

		} else {
			if ( false !== strpos( $_SERVER['SCRIPT_FILENAME'], $abspath_fix ) ) {
				// Request is hitting a file inside ABSPATH.
				$directory = str_replace( ABSPATH, '', $script_filename_dir );
				// Strip off the subdirectory, and any file/query params.
				$path = preg_replace( '#/' . preg_quote( $directory, '#' ) . '/[^/]*$#i', '', $_SERVER['REQUEST_URI'] );
			} elseif ( false !== strpos( $abspath_fix, $script_filename_dir ) ) {
				// Request is hitting a file above ABSPATH.
				$subdirectory = substr( $abspath_fix, strpos( $abspath_fix, $script_filename_dir ) + strlen( $script_filename_dir ) );
				// Strip off any file/query params from the path, appending the subdirectory to the installation.
				$path = preg_replace( '#/[^/]*$#i', '', $_SERVER['REQUEST_URI'] ) . $subdirectory;
			} else {
				$path = $_SERVER['REQUEST_URI'];
			}
		}

		$schema = is_ssl() ? 'https://' : 'http://'; // set_url_scheme() is not defined yet.
		$url    = $schema . $_SERVER['HTTP_HOST'] . $path;
	}

	return rtrim( $url, '/' );
}