<?php
$s = 'Files("textfile1.txt", 7268474425, "textfile2.txt", 661204928, "textfile3.txt", 121034)';

if (preg_match_all('/(?:\w+\(\h*|(?<!\A)\G\h*,\h*)"(?<filename>[^"]+)"\h*,\h*(?<filesize>\d+)/', $s, $m)) {
	$out = array_combine ( $m['filename'], $m['filesize'] );
	print_r($out);
}
?>