Allow objects with __toString in IDType

src/Type/Definition/IDType.php

@@ -43,7 +43,7 @@ public function serialize($value)
         if ($value === null) {
             return 'null';
         }
-        if (!is_scalar($value)) {
+        if (!is_scalar($value) && (!is_object($value) || !method_exists($value, '__toString'))) {
             throw new InvariantViolation("ID type cannot represent non scalar value: " . Utils::printSafe($value));
         }
         return (string) $value;

tests/Type/ObjectIdStub.php

@@ -0,0 +1,23 @@
+<?php
+namespace GraphQL\Tests\Type;
+
+class ObjectIdStub
+{
+	/**
+	 * @var int
+	 */
+	private $id;
+
+	/**
+	 * @param int $id
+	 */
+	public function __construct($id)
+	{
+		$this->id = $id;
+	}
+
+	public function __toString()
+	{
+		return (string) $this->id;
+	}
+}

tests/Type/ScalarSerializationTest.php

@@ -178,4 +178,25 @@ public function testSerializesOutputBoolean()
 
         // TODO: how should it behave on '0'?
     }
+
+    public function testSerializesOutputID()
+    {
+        $idType = Type::id();
+
+        $this->assertSame('string', $idType->serialize('string'));
+        $this->assertSame('', $idType->serialize(''));
+        $this->assertSame('1', $idType->serialize('1'));
+        $this->assertSame('1', $idType->serialize(1));
+        $this->assertSame('0', $idType->serialize(0));
+        $this->assertSame('true', $idType->serialize(true));
+        $this->assertSame('false', $idType->serialize(false));
+        $this->assertSame('2', $idType->serialize(new ObjectIdStub(2)));
+
+        try {
+            $idType->serialize(new \stdClass());
+            $this->fail('Expected exception was not thrown');
+        } catch (InvariantViolation $e) {
+            $this->assertEquals('ID type cannot represent non scalar value: instance of stdClass', $e->getMessage());
+        }
+    }
 }