Add initial support for PHP8

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

Lib/cdata.i

@@ -21,7 +21,7 @@ typedef struct SWIGCDATA {
 }
 %typemap(in) (const void *indata, int inlen) = (char *STRING, int LENGTH);
 
-#elif SWIGPHP7
+#elif SWIGPHP
 
 %typemap(out) SWIGCDATA {
   ZVAL_STRINGL($result, $1.data, $1.len);

Lib/exception.i

@@ -12,7 +12,7 @@
 %insert("runtime") "swigerrors.swg"
 
 
-#ifdef SWIGPHP7
+#ifdef SWIGPHP
 %{
 #include "zend_exceptions.h"
 #define SWIG_exception(code, msg) do { zend_throw_exception(NULL, (char*)msg, code); goto thrown; } while (0)

Lib/php/phprun.swg

@@ -12,8 +12,8 @@ extern "C" {
 #include "zend_exceptions.h"
 #include "php.h"
 
-#if PHP_MAJOR_VERSION != 7
-# error These bindings need PHP7 - to generate PHP5 bindings use: SWIG < 4.0.0 and swig -php5
+#if PHP_MAJOR_VERSION < 7
+# error These bindings need PHP 7 or later - to generate PHP5 bindings use: SWIG < 4.0.0 and swig -php5
 #endif
 
 #include "ext/standard/php_string.h"
@@ -200,7 +200,11 @@ SWIG_ConvertPtr(zval *z, void **ptr, swig_type_info *ty, int flags) {
 
   switch (Z_TYPE_P(z)) {
     case IS_OBJECT: {
+#if PHP_MAJOR_VERSION < 8
       HashTable * ht = Z_OBJ_HT_P(z)->get_properties(z);
+#else
+      HashTable * ht = Z_OBJ_HT_P(z)->get_properties(Z_OBJ_P(z));
+#endif
       if (ht) {
         zval * _cPtr = zend_hash_str_find(ht, "_cPtr", sizeof("_cPtr") - 1);
         if (_cPtr) {