diff --git a/library/HTMLPurifier/ConfigSchema/Interchange.php b/library/HTMLPurifier/ConfigSchema/Interchange.php
index 1ae4330b..4fbde069 100644
--- a/library/HTMLPurifier/ConfigSchema/Interchange.php
+++ b/library/HTMLPurifier/ConfigSchema/Interchange.php
@@ -32,7 +32,7 @@ class HTMLPurifier_ConfigSchema_Interchange
      * Adds a directive array to $directives
      */
     public function addDirective($directive) {
-        if (isset($this->directives[$i = "{$directive->id}"])) {
+        if (isset($this->directives[$i = $directive->id->toString()])) {
             throw new HTMLPurifier_ConfigSchema_Exception("Cannot redefine directive '$i'");
         }
         $this->directives[$i] = $directive;
diff --git a/library/HTMLPurifier/ConfigSchema/Interchange/Id.php b/library/HTMLPurifier/ConfigSchema/Interchange/Id.php
index bb1d6ff3..40a5fe3c 100644
--- a/library/HTMLPurifier/ConfigSchema/Interchange/Id.php
+++ b/library/HTMLPurifier/ConfigSchema/Interchange/Id.php
@@ -13,7 +13,11 @@ class HTMLPurifier_ConfigSchema_Interchange_Id
         $this->directive = $directive;
     }
     
-    public function __toString() {
+    /**
+     * @warning This is NOT magic, to ensure that people don't abuse SPL and
+     *          cause problems for PHP 5.0 support.
+     */
+    public function toString() {
         return $this->namespace . '.' . $this->directive;
     }
     
diff --git a/library/HTMLPurifier/ConfigSchema/InterchangeBuilder.php b/library/HTMLPurifier/ConfigSchema/InterchangeBuilder.php
index 4723b1d5..2b2c9ab4 100644
--- a/library/HTMLPurifier/ConfigSchema/InterchangeBuilder.php
+++ b/library/HTMLPurifier/ConfigSchema/InterchangeBuilder.php
@@ -43,20 +43,21 @@ class HTMLPurifier_ConfigSchema_InterchangeBuilder
         
         // These are required elements:
         $directive->id = $this->id($hash->offsetGet('ID'));
+        $id = $directive->id->toString(); // convenience
         
         if (isset($hash['TYPE'])) {
             $type = explode('/', $hash->offsetGet('TYPE'));
             if (isset($type[1])) $directive->typeAllowsNull = true;
             $directive->type = $type[0];
         } else {
-            throw new HTMLPurifier_ConfigSchema_Exception("TYPE in directive hash '{$directive->id}' not defined");
+            throw new HTMLPurifier_ConfigSchema_Exception("TYPE in directive hash '$id' not defined");
         }
         
         if (isset($hash['DEFAULT'])) {
             try {
                 $directive->default = $this->varParser->parse($hash->offsetGet('DEFAULT'), $directive->type, $directive->typeAllowsNull);
             } catch (HTMLPurifier_VarParserException $e) {
-                throw new HTMLPurifier_ConfigSchema_Exception($e->getMessage() . " in TYPE/DEFAULT in directive hash '{$directive->id}'");
+                throw new HTMLPurifier_ConfigSchema_Exception($e->getMessage() . " in TYPE/DEFAULT in directive hash '$id'");
             }
         }
         
@@ -79,7 +80,7 @@ class HTMLPurifier_ConfigSchema_InterchangeBuilder
                         $this->varParser->parse($real, $directive->type, $directive->typeAllowsNull);
                 }
             } catch (HTMLPurifier_VarParserException $e) {
-                throw new HTMLPurifier_ConfigSchema_Exception($e->getMessage() . " in $alias => $real in VALUE-ALIASES in directive hash '{$directive->id}'");
+                throw new HTMLPurifier_ConfigSchema_Exception($e->getMessage() . " in $alias => $real in VALUE-ALIASES in directive hash '$id'");
             }
         }
         
diff --git a/library/HTMLPurifier/ConfigSchema/Validator.php b/library/HTMLPurifier/ConfigSchema/Validator.php
index c7b29f37..5130ad52 100644
--- a/library/HTMLPurifier/ConfigSchema/Validator.php
+++ b/library/HTMLPurifier/ConfigSchema/Validator.php
@@ -40,7 +40,8 @@ class HTMLPurifier_ConfigSchema_Validator
             $this->validateNamespace($namespace);
         }
         foreach ($interchange->directives as $i => $directive) {
-            if ($i != "{$directive->id}") $this->error(false, "Integrity violation: key '$i' does not match internal id '{$directive->id}'");
+            $id = $directive->id->toString();
+            if ($i != $id) $this->error(false, "Integrity violation: key '$i' does not match internal id '$id'");
             $this->validateDirective($directive);
         }
     }
@@ -57,7 +58,8 @@ class HTMLPurifier_ConfigSchema_Validator
     }
     
     public function validateId($id) {
-        $this->context[] = "id '$id'";
+        $id_string = $id->toString();
+        $this->context[] = "id '$id_string'";
         if (!$id instanceof HTMLPurifier_ConfigSchema_Interchange_Id) {
             // handled by InterchangeBuilder
             $this->error(false, 'is not an instance of HTMLPurifier_ConfigSchema_Interchange_Id');
@@ -72,7 +74,8 @@ class HTMLPurifier_ConfigSchema_Validator
     }
     
     public function validateDirective($d) {
-        $this->context[] = "directive '{$d->id}'";
+        $id = $d->id->toString();
+        $this->context[] = "directive '$id'";
         $this->validateId($d->id);
         $this->with($d, 'description')
             ->assertNotEmpty();
diff --git a/tests/HTMLPurifier/ConfigSchema/ValidatorTest.php b/tests/HTMLPurifier/ConfigSchema/ValidatorTest.php
index 78456bc4..f5fa49e0 100644
--- a/tests/HTMLPurifier/ConfigSchema/ValidatorTest.php
+++ b/tests/HTMLPurifier/ConfigSchema/ValidatorTest.php
@@ -40,20 +40,6 @@ class HTMLPurifier_ConfigSchema_ValidatorTest extends UnitTestCase
         $this->validator->validate($this->interchange);
     }
     
-    public function testDirectiveIdInstanceof() {
-        // This example is somewhat contrived
-        $this->makeNamespace('Ns');
-        $d = new HTMLPurifier_ConfigSchema_Interchange_Directive();
-        $d->id = 3;
-        $d->default = 0;
-        $d->type = 'int';
-        $d->description = 'Description';
-        $this->interchange->addDirective($d);
-        
-        $this->expectValidationException("Id '3' in directive '3' is not an instance of HTMLPurifier_ConfigSchema_Interchange_Id");
-        $this->validator->validate($this->interchange);
-    }
-    
     public function testDirectiveTypeNotEmpty() {
         $this->makeNamespace('Ns');
         $d = $this->makeDirective('Ns', 'Dir');