Support object->exists() (#453)

* Support object->exists()

* matches js sdk

* Improve documentation

Author: dplewis

src/Parse/ParseObject.php

@@ -306,6 +306,30 @@ function ($object) use (&$result) {
         return $result;
     }
 
+    /**
+     * Returns true if this object exists on the Server
+     *
+     * @param bool $useMasterKey Whether to use the Master Key.
+     *
+     * @return bool
+     */
+    public function exists($useMasterKey = false)
+    {
+        if (!$this->objectId) {
+            return false;
+        }
+        try {
+            $query = new ParseQuery($this->className);
+            $query->get($this->objectId, $useMasterKey);
+            return true;
+        } catch (Exception $e) {
+            if ($e->getCode() === 101) {
+                return false;
+            }
+            throw $e;
+        }
+    }
+
     /**
      * Validate and set a value for an object key.
      *

tests/Parse/ParseObjectTest.php

@@ -1775,4 +1775,19 @@ public function testUnrecognizedOp()
         ];
         ParseObject::decode($encoded);
     }
+
+    /**
+     * Tests if object exists
+     *
+     * @group object-exists
+     */
+    public function testObjectExists()
+    {
+        $obj = new ParseObject('TestClass');
+        $this->assertEquals($obj->exists(), false);
+        $obj->save();
+        $this->assertEquals($obj->exists(), true);
+        $obj->destroy();
+        $this->assertEquals($obj->exists(), false);
+    }
 }