There are classes and interfaces that can be either user defined or pre-defines in PHP language. 

There are 9 predefined classes and interfaces in PHP:

• Traversable
• Iterator 
• IteratorAggregate
• Throwtable
• ArrayAccess
• Serializable
• Closure
• Generator
   

Traversable: There are no methods in the Traversable interface and acts only as a base for other predefined Traversable classes. By using foreach construct and this interface, it is easily detectable whether the class is traversable. Iterator: This is a predefined interface for objects or external iterators that are capable of internally iterating themselves. This interface extends the interface Traversable. The five methods in this interface are:

Method name	parameters	return value	description
current	void	mixed	The present value is returned.
key	void	scalar	The present value's key is returned.
next	void	void	Moving forward to the next element.
rewind	void	void	Set the position of the file pointer to the beginning of the file.
valid	void	bool	Check whether an array contains more entries or not.

IteratorAggregate: This interface is an extension of Traversable interface and has one method.

method name	parameters	return type	description
getIterator	void	traversable	This function is used for retrieving an iterator(external).

Throwtable

This feature is available in PHP version 7. All objects that are thrown through the throw statement has an interface which acts as base interface called Throwtable. Exception and Error classes use this interface.

Method name	number of arguments	name of arguments	return type	description
getMessage	0	void	string	Returns the message to be displayed when exception occurs.
getPrevious	0	void	throwtable	Previous exceptions are returned.
getCode	0	void	mixed	The code for exception generated.
getFile	0	void	string	The file in which exception occurs is returned.
getLine	0	void	int	The line number in which exception occurs returned.
getTrace	0	void	array	The stack trace is returned.
getTraceAsString	0	void	string	The stack trace is returned in the form of string.
__toString	0	void	string	The exception generated is returned in the form of string.

ArrayAccess: This interface allows objects to be accessed as an arrays. It has 4 methods:

method name	parameters	returns	description
offsetExists	offset value(mixed)	bool	Checks existence of the offset.
offsetGet	offset value(mixed)	mixed	The offset is retrieved through this method.
offsetSet	offset value(mixed), element(mixed)	void	A mentioned offset is given some value.
offsetUnset	offset value(mixed)	void	The value of offset is unset.

Serializable: This class allows serialization in a customized fashion. If serialization of an instance is needed then the method serialize() is called. If it is written in the code inside some method there are no alterations or side effects to the program. There are two methods in this interface:

method name	parameters	return	description
serialize	void	string	Object is converted into string.
unserialize	serialized string (string)	void	The object that was converted into string is converted back to an object.

Closure: There are some functions called anonymous function in PHP that have no name. The callback parameters are assigned the value as anonymous function. There are 5 methods:

method name	parameters	returns	description
__construct	void	void	A constructor disallowing instantiation
bind	object (closure) newthis (object)closure	closure	A closure can be duplicated by the class scope and mentioned particular bound object.
bindTo	newthis (object)closure	closure	A closure can be duplicated by the class scope and new bound object.
call	newthis (object)closure	mixed	The closure is binded and called.
fromCallable	Callable (Callable )	closure	The callable is converted into closure.

Generator: This interface is extension of Iterator and has 9 methods:

method name	parameters	returns	descitption
current	void	mixed	The yielded element can be found.
getReturn	void	mixed	The generator's value is returned by the function.
key	void	mixed	The key which was yielded is returned.
next	void	void	The generator's execution is resumed.
rewind	void	void	The iterator is rewinded.
send	value(mixed)	mixed	A value is sent to the generator.
throw	exception(throwtable)	mixed	An exception is thrown into the generator.
valid	void	bool	Verifies if there was a closing of the iterator.
__wakeup	void	void	Callback is serialized.