It seems to be a PHP-serialized empty array, base 64 encoded.
$ base64 -D <<< 'YTowOnt9'
a:0:{}
$ php -r 'var_dump(unserialize(base64_decode("YTowOnt9")));'
array(0) {
}
There are many scripts that serialize arrays of data. When the arrays have data, they vary greatly, so the Base64 encoded PHP-serialized values do too, but when they are empty they are all the same. It makes it look as if a lot of very different PHP scripts have this random string in common.