A userland implementation of a generator function. A bit like a python context manager too.

php-generator-example.php

<?php

namespace Functional;

error_reporting(E_ALL|E_STRICT);

const BEFORE = 1;
const NEXT = 2;
const AFTER = 3;
const FORWARD = 4;
const YIELD = 5;

class Generator implements \Iterator {
    private $funcs;
    private $args;
    private $key;
    private $result;
    
    public function __construct(array $funcs, array $args) {
        $this->funcs = $funcs;
        $this->args = $args;
    }
    
    public function rewind() {
        $this->key = -1;
        $this->result = call_user_func_array($this->funcs[BEFORE], 
                                             $this->args);
        $this->next();
    }
 
    public function valid() {
        return $this->result[YIELD] !== false;
    }
 
    public function current() {
        return $this->result[YIELD];
    }
 
    public function key() {
        return $this->key;
    }
 
    public function next() {
        $this->result = call_user_func($this->funcs[NEXT], 
                                       $this->result[FORWARD]);
        if ($this->result[YIELD] === false) {
            call_user_func($this->funcs[AFTER], $this->result[FORWARD]);
        }
        ++$this->key;
    }
}

/**
 * A generator function that lazily yields each line in a file.
 */
function get_lines_from_file($file_name) {
    $funcs = array(
        BEFORE => function($file_name) { 
            return array(FORWARD => fopen($file_name, 'r')); },
        NEXT   => function($fh) { 
            return array(FORWARD => $fh, YIELD => fgets($fh)); },
        AFTER  => function($fh) { fclose($fh); },
    );
    return new Generator($funcs, array($file_name));
}

// Output content of this file with padded linenumbers.
foreach (get_lines_from_file(__FILE__) as $k => $v) {
    echo str_pad($k, 8), $v;
}
echo "\n";