<?php

function fib() {
    [$x, $y] = [0, 1];
    while (true) {
        yield $x;
        [$x, $y] = [$y, $x + $y];
    }
}

for ($f = fib(), $i = 0; $i < 10; $i++) {
    echo $f->current() . PHP_EOL;
    $f->next();
}