PHP_CompatInfo can help with a few checks for dependencies. PHP_CodeSniffer maybe with finding outdated constructs.
However, the differences between PHP versions are often gloryfied. Never had much problems with code relying on fringe cases. Unless of course this code still relies on long superglobals $HTTP_POST_VARS, then a few other gotchas are to be expected.
A helpful idiom to replace magic_quotes dependency for example is:
$_POST = array_map("mysql_real_escape_string", $_POST);
Because realistically most applications are never rewritten to use more contemporary database APIs.
