Trying to replace parts of string start with same search chars

Besides that you should define your array first before you use it, this should work for you:

$str = strtr($string, $my_array);

Your problem is that str_replace() goes through the entire string and replaces everything it can, you can also see this in the manual.

And a quote from there:

Because str_replace() replaces left to right, it might replace a previously inserted value when doing multiple replacements. See also the examples in this document.

So for this I used strtr() here, because it tries to match the longest byte in the search first.

You can also read this in the manual and a quote from there:

If given two arguments, the second should be an array in the form array('from' => 'to', ...). The return value is a string where all the occurrences of the array keys have been replaced by the corresponding values. The longest keys will be tried first. Once a substring has been replaced, its new value will not be searched again.

Try to replace first for :y2 and then for :y

$string = "Good one :y. Keep going :y2"; 

$my_array= array(":y2" => "b", ":y" => "a");

$str = str_replace(array_keys($my_array), array_values($my_array), $string);

outputs

Good one a. Keep going b

Try it