PHP concatenation override [on hold]

Is it possible to change a concatened string, with a variable in the string?

  $x = 'XXX' . $y;

Is there a way to have $x contain 'XXY', without changing this?

What do I need to set in $y for this? Does there exist something like a "remove previous character"?

EDIT:

Maybe i didnt made myself crystal clear:

$y needs to be a string, no functions or anything. Its due to discover an exploit...