<?php
class IT extends ArrayIterator {
    private $n = 0;

    function __construct($trap = null) {
        parent::__construct([0, 1]);
        $this->trap = $trap;
    }

    function trap($trap) {
        if ($trap === $this->trap) {
            throw new Exception($trap);
        }
    }

    function rewind(): void  {$this->trap(__FUNCTION__); parent::rewind();}
    function valid(): bool   {$this->trap(__FUNCTION__); return parent::valid();}
    function key(): string|int|null { $this->trap(__FUNCTION__); return parent::key(); }
    function next(): void    {$this->trap(__FUNCTION__); parent::next();}
}

foreach(['rewind', 'valid', 'key', 'next'] as $trap) {
    $obj = new IT($trap);
    try {
        // IS_CV
        foreach ($obj as $key => &$val) echo "$val\n";
    } catch (Exception $e) {
        echo $e->getMessage() . "\n";
    }
    unset($obj);

    try {
        // IS_VAR
        foreach (new IT($trap) as $key => &$val) echo "$val\n";
    } catch (Exception $e) {
        echo $e->getMessage() . "\n";
    }

    try {
        // IS_TMP_VAR
        foreach ((object)new IT($trap) as $key => &$val) echo "$val\n";
    } catch (Exception $e) {
        echo $e->getMessage() . "\n";
    }
}
?>