Source

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

public function delete( $file, $recursive = false, $type = false ) {
		if ( empty( $file ) ) {
			// Some filesystems report this as /, which can cause non-expected recursive deletion of all files in the filesystem.
			return false;
		}

		$file = str_replace( '\\', '/', $file ); // For Win32, occasional problems deleting files otherwise.

		if ( 'f' === $type || $this->is_file( $file ) ) {
			return @unlink( $file );
		}

		if ( ! $recursive && $this->is_dir( $file ) ) {
			return @rmdir( $file );
		}

		// At this point it's a folder, and we're in recursive mode.
		$file     = trailingslashit( $file );
		$filelist = $this->dirlist( $file, true );

		$retval = true;

		if ( is_array( $filelist ) ) {
			foreach ( $filelist as $filename => $fileinfo ) {
				if ( ! $this->delete( $file . $filename, $recursive, $fileinfo['type'] ) ) {
					$retval = false;
				}
			}
		}

		if ( file_exists( $file ) && ! @rmdir( $file ) ) {
			$retval = false;
		}

		return $retval;
	}