Add mixed type

"mixed" type declare is added in PHP 8.0.

https://wiki.php.net/rfc/mixed_type_v2

Author: zonuexe

php-mode.el

@@ -472,7 +472,7 @@ In that case set to `NIL'."
 
 (c-lang-defconst c-primitive-type-kwds
   php '("int" "integer" "bool" "boolean" "float" "double" "real"
-        "string" "object" "void"))
+        "string" "object" "void" "mixed"))
 
 (c-lang-defconst c-class-decl-kwds
   "Keywords introducing declarations where the following block (if any)

tests/lang/types/keywords.php

@@ -12,3 +12,4 @@
 string;
 object;
 resource;
+mixed;