[php] Fix testcase segfaults with PHP 8.0

These testcases were segfaulting:

prefix
director_using_member_scopes
virtual_poly

The fix here is admittedly a hack - we perform the initialisation
of EG(class_table) from CG(class_table) which PHP will do, but
hasn't yet.

PHP doesn't seem to clearly document which API calls are actually
valid in minit or other initialisation contexts, but the code we're
generating works with all PHP 7.x and PHP 8.x versions aside from PHP
8.0 so it seems this is a bug in PHP 8.0 rather than that we're doing
something invalid, and we need to work with existing PHP 8.0 releases
so this hack seems a necessary evil.  It will at least have a limited
life as PHP 8.0 is only in active support until 2022-11-26, with
security support ending a year later.

Fixes #2383.

Lib/php/phpinit.swg

@@ -9,4 +9,8 @@
 SWIG_php_minit {
   zend_class_entry SWIGUNUSED internal_ce;
   SWIG_InitializeModule((void*)&module_number);
+#if PHP_MAJOR_VERSION == 8 && PHP_MINOR_VERSION == 0
+  /* This hack is needed to avoid segfaults. */
+  EG(class_table) = CG(class_table);
+#endif
 %}