diff --git a/Zend/tests/bug53727.phpt b/Zend/tests/bug53727.phpt
new file mode 100644
index 00000000000..22cd5232c7c
--- /dev/null
+++ b/Zend/tests/bug53727.phpt
@@ -0,0 +1,22 @@
+--TEST--
+Bug #53727 (Inconsistent behavior of is_subclass_of with interfaces)
+--FILE--
+<?php
+interface MyInterface {
+    const TEST_CONSTANT = true;
+}
+
+class ParentClass implements MyInterface { }
+
+class ChildClass extends ParentClass { }
+
+echo (is_subclass_of('ChildClass', 'MyInterface') ? 'true' : 'false') . "\n";
+echo (defined('ChildClass::TEST_CONSTANT') ? 'true' : 'false') . "\n";
+
+echo (is_subclass_of('ParentClass', 'MyInterface') ? 'true' : 'false') . "\n";
+echo (defined('ParentClass::TEST_CONSTANT') ? 'true' : 'false') . "\n";
+--EXPECT--
+true
+true
+true
+true
diff --git a/Zend/tests/is_a.phpt b/Zend/tests/is_a.phpt
index a074194c4d4..0a01eb756e1 100755
--- a/Zend/tests/is_a.phpt
+++ b/Zend/tests/is_a.phpt
@@ -38,6 +38,6 @@ bool(true)
 bool(false)
 bool(false)
 bool(true)
-bool(false)
+bool(true)
 AUTOLOAD 'X1'
 bool(false)
diff --git a/Zend/zend_builtin_functions.c b/Zend/zend_builtin_functions.c
index 8b96f4729a1..b634e3de00c 100644
--- a/Zend/zend_builtin_functions.c
+++ b/Zend/zend_builtin_functions.c
@@ -845,45 +845,26 @@ static void is_a_impl(INTERNAL_FUNCTION_PARAMETERS, zend_bool only_subclass)
 		return;
 	}
 	
-	if (only_subclass && Z_TYPE_P(obj) == IS_STRING) {
+	if (Z_TYPE_P(obj) == IS_STRING) {
 		zend_class_entry **the_ce;
 		if (zend_lookup_class(Z_STRVAL_P(obj), Z_STRLEN_P(obj), &the_ce TSRMLS_CC) == FAILURE) {
 			zend_error(E_WARNING, "Unknown class passed as parameter");
 			RETURN_FALSE;
 		}
 		instance_ce = *the_ce;
-	} else if (Z_TYPE_P(obj) != IS_OBJECT) {
-		RETURN_FALSE;
+	} else if (Z_TYPE_P(obj) == IS_OBJECT && HAS_CLASS_ENTRY(*obj)) {
+		instance_ce = Z_OBJCE_P(obj);
 	} else {
-		instance_ce = NULL;
-	}
-
-	/* TBI!! new object handlers */
-	if (Z_TYPE_P(obj) == IS_OBJECT && !HAS_CLASS_ENTRY(*obj)) {
 		RETURN_FALSE;
 	}
 
 	if (zend_lookup_class_ex(class_name, class_name_len, NULL, 0, &ce TSRMLS_CC) == FAILURE) {
 		retval = 0;
 	} else {
-		if (only_subclass) {
-			if (!instance_ce) {
-				instance_ce = Z_OBJCE_P(obj)->parent;
-			} else {
-				instance_ce = instance_ce->parent;
-			}
-		} else {
-			instance_ce = Z_OBJCE_P(obj);
-		}
-
-		if (!instance_ce) {
-			RETURN_FALSE;
- 		}
-
-		if (instanceof_function(instance_ce, *ce TSRMLS_CC)) {
-			retval = 1;
-		} else {
+		if (only_subclass && instance_ce == *ce) {
 			retval = 0;
+ 		} else {
+			retval = instanceof_function(instance_ce, *ce TSRMLS_CC);
 		}
 	}
 
diff --git a/ext/standard/tests/class_object/is_a_variation_001.phpt b/ext/standard/tests/class_object/is_a_variation_001.phpt
index d2d6ce24071..1021544065b 100644
--- a/ext/standard/tests/class_object/is_a_variation_001.phpt
+++ b/ext/standard/tests/class_object/is_a_variation_001.phpt
@@ -144,15 +144,23 @@ Arg value
 bool(false)
 
 Arg value  
+
+Warning: Unknown class passed as parameter in %sis_a_variation_001.php on line %d
 bool(false)
 
 Arg value  
+
+Warning: Unknown class passed as parameter in %sis_a_variation_001.php on line %d
 bool(false)
 
 Arg value string 
+
+Warning: Unknown class passed as parameter in %sis_a_variation_001.php on line %d
 bool(false)
 
 Arg value String 
+
+Warning: Unknown class passed as parameter in %sis_a_variation_001.php on line %d
 bool(false)
 
 Arg value