Add initial support for PHP8

Testcase director_overload2 is failing, but the rest of the testsuite
passes.

Examples/test-suite/php/evil_diamond_prop_runme.php

@@ -31,7 +31,7 @@ check::is_a($spam,"spam");
 check::equal(1,$spam->_foo,"1==spam->_foo");
 check::equal(2,$spam->_bar,"2==spam->_bar");
 // multiple inheritance not supported in PHP
-set_error_handler(NULL, 0); // Don't complain that _baz is unknown.
+set_error_handler(function () {return true;}, E_NOTICE|E_WARNING); // Don't complain that _baz is unknown.
 check::equal(null,$spam->_baz,"null==spam->_baz");
 restore_error_handler();
 check::equal(4,$spam->_spam,"4==spam->_spam");