why-when-generator-php-blog-gist1.php

<?php

class FileIterator implements \Iterator {
    protected $filePointer;
    protected $data;
    protected $key;

    public function __construct($file) {
        $this->filePointer = fopen($file, 'rb');
        if (!$this->filePointer) {
            throw new \Exception('File could not be opened');
        };
    }
    public function __destruct() {
        fclose($this->filePointer);
    }
    public function current() {
        return $this->data;
    }
    public function key() {
        return $this->key;
    }
    public function next(): void {
        $this->data = fgets($this->filePointer);
        $this->key++;
    }
    public function rewind(): void {
        fseek($this->filePointer, 0);
        $this->data = fgets($this->filePointer);
        $this->key = 0;
    }
    public function valid(): bool {
        return false !== $this->data;
    }
}

$lines = new FileIterator('/path-to-file');
foreach ($lines as $line) {
    echo $line. "\n";
}

good question @geshan. At the start, rewind is called, where the key is set to 0.