Update TConstant.php to remove deprecated PHP code

remove deprecated use of static callables
diff --git a/lib/php/lib/Type/TConstant.php b/lib/php/lib/Type/TConstant.php
index 215da4a..9e3dd62 100644
--- a/lib/php/lib/Type/TConstant.php
+++ b/lib/php/lib/Type/TConstant.php
@@ -41,12 +41,12 @@
      */
     public static function get($constant)
     {
-        if (is_null(static::$$constant)) {
-            static::$$constant = call_user_func(
-                sprintf('static::init_%s', $constant)
+        if (is_null(static::class::$$constant)) {
+            static::class::$$constant = call_user_func(
+                sprintf(static::class . '::init_%s', $constant)
             );
         }
 
-        return static::$$constant;
+        return static::class::$$constant;
     }
 }