Source

File: wp-admin/includes/class-wp-filesystem-base.php

public function search_for_folder( $folder, $base = '.', $loop = false ) {
		if ( empty( $base ) || '.' === $base ) {
			$base = trailingslashit( $this->cwd() );
		}

		$folder = untrailingslashit( $folder );

		if ( $this->verbose ) {
			/* translators: 1: Folder to locate, 2: Folder to start searching from. */
			printf( "\n" . __( 'Looking for %1$s in %2$s' ) . "<br/>\n", $folder, $base );
		}

		$folder_parts     = explode( '/', $folder );
		$folder_part_keys = array_keys( $folder_parts );
		$last_index       = array_pop( $folder_part_keys );
		$last_path        = $folder_parts[ $last_index ];

		$files = $this->dirlist( $base );

		foreach ( $folder_parts as $index => $key ) {
			if ( $index == $last_index ) {
				continue; // We want this to be caught by the next code block.
			}

			/*
			 * Working from /home/ to /user/ to /wordpress/ see if that file exists within
			 * the current folder, If it's found, change into it and follow through looking
			 * for it. If it can't find WordPress down that route, it'll continue onto the next
			 * folder level, and see if that matches, and so on. If it reaches the end, and still
			 * can't find it, it'll return false for the entire function.
			 */
			if ( isset( $files[ $key ] ) ) {

				// Let's try that folder:
				$newdir = trailingslashit( path_join( $base, $key ) );

				if ( $this->verbose ) {
					/* translators: %s: Directory name. */
					printf( "\n" . __( 'Changing to %s' ) . "<br/>\n", $newdir );
				}

				// Only search for the remaining path tokens in the directory, not the full path again.
				$newfolder = implode( '/', array_slice( $folder_parts, $index + 1 ) );
				$ret       = $this->search_for_folder( $newfolder, $newdir, $loop );

				if ( $ret ) {
					return $ret;
				}
			}
		}

		// Only check this as a last resort, to prevent locating the incorrect install.
		// All above procedures will fail quickly if this is the right branch to take.
		if ( isset( $files[ $last_path ] ) ) {
			if ( $this->verbose ) {
				/* translators: %s: Directory name. */
				printf( "\n" . __( 'Found %s' ) . "<br/>\n", $base . $last_path );
			}

			return trailingslashit( $base . $last_path );
		}

		// Prevent this function from looping again.
		// No need to proceed if we've just searched in `/`.
		if ( $loop || '/' === $base ) {
			return false;
		}

		// As an extra last resort, Change back to / if the folder wasn't found.
		// This comes into effect when the CWD is /home/user/ but WP is at /var/www/....
		return $this->search_for_folder( $folder, '/', true );

	}