how to fix Warning: Use of undefined constant _ – assumed ‘_’ (this will throw an Error in a future version of PHP)

Problem lies in this line:

$filename = $acak._.$file; 

It should be

$filename = $acak.'_'.$file; 

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)