<?php

function f1() {

    for ($i = 0 ; $i < 21 ; $i++) {
        yield $i;
    }

}


$generator = f1();

echo "<pre>";

echo $generator->current();
$generator->next();

echo "\n";

echo $generator->current();

echo "\n";

echo $generator->current();

echo "\n";

do {
    echo $generator->current();
    $generator->next();
    echo "\n";
} while ($generator->valid());

echo "</pre>";