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 );