Shows the behaviour of array pointer functions called on custom Iterator objects

iterator-quirks.php

<?php
/**
 * A warning to myself that custom iterators cannot entirely masquerade as native arrays for legacy code compatibility:
 * 
 * - Custom classes implementing the Iterator interface work with `foreach`, but not with native array pointer functions.
 * - Built-in iterators do work with `current`, `key` and `next` but the iterator methods cannot be overridden in subclasses.
 */


/**
 * Entirely user-defined iterator class
 */
class CustomIterator implements Iterator {

    private $a;
    
    public function __construct( array $a = [] ){
        $this->a = $a;
    }

    public function rewind(){
        reset( $this->a );
    }

    public function key(){
        return key( $this->a );
    }

    public function current(){
        return current( $this->a );
    }

    public function valid(){
        return is_int( key($this->a) );
    }

    public function next(){
        next( $this->a );
    }    

}


/**
 * Extending built-in iterator
 */
class ExtendedIterator extends ArrayIterator {
    
    public function next(){
        throw new Exception("You won't see this");
    }

    public function current(){
        throw new Exception("You won't see this");
    }
    
}



// Built-in class implementing Iterator
$test = new ArrayIterator( ['Skip this', ':-) all good'] );
next($test);
echo "Native ",current($test),"\n";


// Extending built-in functions
$test = new ExtendedIterator( ['Skip this', ':-/ overrides ignored'] );
next($test);
echo "Extend ",current($test),"\n";


// Completely custom user-defined class:
$test = new CustomIterator( ['Foo', 'Bar'] );
next($test);
echo "Custom ",current($test)?:":-( Just don't.","\n";


// Prints:
// Native :-) all good
// Extend :-/ overrides ignored
// Custom :-( Just don't.