How can I remove 3 characters at the end of a string in php?

Just do:

echo substr($string, 0, -3);

You don't need to use a strlen call, since, as noted in the substr docs:

If length is given and is negative, then that many characters will be omitted from the end of string

<?php echo substr("abcabcabc", 0, -3); ?>

<?php echo substr($string, 0, strlen($string) - 3); ?>