I use token_get_all for this. I have some PHP code in the db. Before saving, I do

function is_valid_php_code_or_throw( $code ) {
        $old = ini_set('display_errors', 1);
        try {
                token_get_all("<?php\n$code", TOKEN_PARSE);
        }
        catch ( Throwable $ex ) {
                $error = $ex->getMessage();
                $line = $ex->getLine() - 1;
                throw new InvalidInputException("PARSE ERROR on line $line:\n\n$error");
        }
        finally {
                ini_set('display_errors', $old);
        }
}

Works like a charm. Syntax only. No missing variables, type incompayibility etc.

InvalidInputException is my own. You can make it anything, or return a bool, or handle the exception yourself.

I'm not sure if display_errors is necessary. It was at some point.