Source

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

public function find_folder( $folder ) {
		if ( isset( $this->cache[ $folder ] ) ) {
			return $this->cache[ $folder ];
		}

		if ( stripos( $this->method, 'ftp' ) !== false ) {
			$constant_overrides = array(
				'FTP_BASE'        => ABSPATH,
				'FTP_CONTENT_DIR' => WP_CONTENT_DIR,
				'FTP_PLUGIN_DIR'  => WP_PLUGIN_DIR,
				'FTP_LANG_DIR'    => WP_LANG_DIR,
			);

			// Direct matches ( folder = CONSTANT/ ).
			foreach ( $constant_overrides as $constant => $dir ) {
				if ( ! defined( $constant ) ) {
					continue;
				}

				if ( $folder === $dir ) {
					return trailingslashit( constant( $constant ) );
				}
			}

			// Prefix matches ( folder = CONSTANT/subdir ),
			foreach ( $constant_overrides as $constant => $dir ) {
				if ( ! defined( $constant ) ) {
					continue;
				}

				if ( 0 === stripos( $folder, $dir ) ) { // $folder starts with $dir.
					$potential_folder = preg_replace( '#^' . preg_quote( $dir, '#' ) . '/#i', trailingslashit( constant( $constant ) ), $folder );
					$potential_folder = trailingslashit( $potential_folder );

					if ( $this->is_dir( $potential_folder ) ) {
						$this->cache[ $folder ] = $potential_folder;

						return $potential_folder;
					}
				}
			}
		} elseif ( 'direct' === $this->method ) {
			$folder = str_replace( '\\', '/', $folder ); // Windows path sanitisation.

			return trailingslashit( $folder );
		}

		$folder = preg_replace( '|^([a-z]{1}):|i', '', $folder ); // Strip out Windows drive letter if it's there.
		$folder = str_replace( '\\', '/', $folder ); // Windows path sanitisation.

		if ( isset( $this->cache[ $folder ] ) ) {
			return $this->cache[ $folder ];
		}

		if ( $this->exists( $folder ) ) { // Folder exists at that absolute path.
			$folder                 = trailingslashit( $folder );
			$this->cache[ $folder ] = $folder;

			return $folder;
		}

		$return = $this->search_for_folder( $folder );

		if ( $return ) {
			$this->cache[ $folder ] = $return;
		}

		return $return;
	}