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Generators produce a sequence of values lazily, one at a time. Use yield instead of return:

function range_gen(int $start, int $end): \Generator {
    for ($i = $start; $i <= $end; $i++) {
        yield $i;
    }
}

foreach (range_gen(1, 5) as $n) {
    echo $n . "\n";
}

range_gen(1, 1_000_000) allocates almost no memory — it generates each value on demand. Compare to range(1, 1_000_000) which allocates a million-element array.

yield with keys:

function wordCounts(string $text): \Generator {
    foreach (explode(' ', $text) as $w) {
        yield $w => strlen($w);
    }
}

foreach (wordCounts('the cat ran') as $word => $len) { ... }

yield from — delegate to another generator (like Python's):

function tree(): \Generator {
    yield 1;
    yield from subtree();    // yields all subtree's values
    yield 99;
}

Common use cases:

• Reading large files line by line: yield $line inside a fgets loop
• Lazy filtering chains
• Pagination — yield each page's items without holding them all

Generators implement Iterator so they work in foreach directly.