Use non-deprecated syntax string interpolation in skip check include

tests/tests-skipcheck.inc.php

@@ -2,33 +2,33 @@
 extension_loaded('timecop') or die('skip timecop module not available');
 if (isset($required_version)) {
     if (version_compare(PHP_VERSION, $required_version, "<")) {
-        die("skip PHP ${required_version}+ required for this test");
+        die("skip PHP {$required_version}+ required for this test");
     }
 }
 if (isset($required_version_less_than)) {
     if (version_compare($required_version_less_than, PHP_VERSION, "<")) {
-        die("skip PHP < ${required_version_less_than} required for this test");
+        die("skip PHP < {$required_version_less_than} required for this test");
     }
 }
 if (isset($required_func)) {
     foreach ($required_func as $func_name) {
         if (!function_exists($func_name)) {
-            die("skip $func_name() function is not available.");
+            die("skip {$func_name}() function is not available.");
         }
     }
 }
 if (isset($required_class)) {
     foreach ($required_class as $class_name) {
         if (!class_exists($class_name)) {
-            die("skip $class_name class is not available.");
+            die("skip {$class_name} class is not available.");
         }
     }
 }
 if (isset($required_method)) {
     foreach ($required_method as $v) {
         list($class_name, $method_name) = $v;
         if (!method_exists($class_name, $method_name)) {
-            die("skip $class_name::$method_name() method is not available.");
+            die("skip {$class_name}::{$method_name}() method is not available.");
         }
     }
 }