private function get_po_revision( $file ) {
		$file        = $this->language_files_dir . '/' . $file;
		$headers     = array( 'PO-Revision-Date' => '"PO-Revision-Date' );
		$all_headers = array();

		$fp = fopen( $file, 'r' );

		// Pull only the first 1kiB of the file in.
		$contents = fread( $fp, 1024 );

		fclose( $fp );

		foreach ( $headers as $field => $regex ) {
			if ( preg_match( '/^[ \t\/*#@]*' . preg_quote( $regex, '/' ) . ':(.*)$/mi', $contents, $match ) && $match[1] ) {
				$value                 = $this->_cleanup_header_comment( $match[1] );
				$value                 = preg_replace( '~(\\\n)?"$~', '', $value );
				$all_headers[ $field ] = $value;
			} else {
				$all_headers[ $field ] = '';
			}
		}

		return $all_headers['PO-Revision-Date'];
	}