Note that it says file name not variable name.
PHP strings definitely can handle the number of characters that you posted, and none of the PHP variable names are even remotely long.
There is some other problem, but I'm not sure what.
btw, enclosing your variable in parentheses isnt really standard practice.
Such use of parentheses is generally for conditionals where there are more than one sub-conditional to be evaluated, e.g.
($a && $b) || ($c && $d)