Assignments must be the first block of code on a line Validation Error on Travis

You’re not supposed to assign multiple variables on a single line. Do them separately:

$pf_param_string = substr( $pf_param_string, 0, - 1 );
$validate_string = $pf_param_string;

Or, if you don’t need both variables, just skip one of them:

$validate_string = substr( $pf_param_string, 0, - 1 );