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en/articles/from-tg/series/php-can-do/generators.md
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# Generators
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*An old tool most people still don't use*
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- **Added in:** PHP 5.5
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- **What it gives you:** functions with `yield` that return elements one at a time β instead of collecting everything in memory and handing it over at once.
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## The reality
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Even though generators have existed in PHP since 5.5, at interviews most developers either can't explain where exactly they save memory, or mention them only in theory β something like "it's a thing for big data". As for practical cases β zero ideas.
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So let me throw you a few ideas on how and where they can be used.
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## How they actually work
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Unlike regular iterators or arrays:
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- a generator doesn't know how many elements there will be;
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- it doesn't know whether there's a next one;
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- it doesn't remember whether there was a previous iteration;
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- it's single-pass β once you've gone through it, that's it, you can't go back.
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And despite all these "limitations" it has one major advantage: it can pause its own execution, which means you don't keep all the states in memory, only the current one.
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It's not an "array" but rather a stream β and that's exactly why it's priceless in the right place.
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## Real cases where yield is truly needed
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### 1. Reading from the database page by page
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```php
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function fetchUsers(PDO $db, int $limit = 500): Generator
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{
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$offset = 0;
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while (true) {
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$rows = $db->query("SELECT * FROM users LIMIT $limit OFFSET $offset")->fetchAll();
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if (!$rows) break;
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yield from $rows;
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$offset += $limit;
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}
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}
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```
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The memory savings are enormous β it works even on tables with hundreds of thousands of rows.
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### 2. Streaming parsing of large files
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```php
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function readCsv(string $path): Generator
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{
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$h = fopen($path, 'r');
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while (($row = fgetcsv($h)) !== false) {
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yield $row;
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}
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fclose($h);
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}
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```
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It reads line by line β the whole CSV never gets loaded into memory.
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### 3. Streaming the result into a file
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```php
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function streamCsvRows(iterable $data): Generator
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{
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foreach ($data as $row) {
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yield implode(',', $row) . "\n";
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}
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}
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header('Content-Type: text/csv');
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foreach (streamCsvRows($repository->getLargeDataset()) as $line) {
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echo $line;
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flush();
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}
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```
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Sending large reports or exports β without buffering, in real time.
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### 4. Loading from an API gradually
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```php
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function fetchPages(ApiClient $client): Generator
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{
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$page = 1;
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do {
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$data = $client->getPage($page++);
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yield from $data['items'];
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} while ($data['has_more']);
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}
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```
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Perfect for streaming data processing without buffering.
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### 5. Walking a large file tree
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```php
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function scanDirRecursive(string $path): Generator
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{
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foreach (scandir($path) as $item) {
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if ($item === '.' || $item === '..') continue;
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$full = "$path/$item";
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if (is_dir($full)) {
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yield from scanDirRecursive($full);
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} else {
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yield $full;
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}
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}
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}
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foreach (scanDirRecursive('/var/log') as $file) {
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echo $file . PHP_EOL;
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}
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```
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Perfect for walking directories, even if there are thousands of them.
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## Conclusion
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> Generators are a tool not for convenience, but for efficiency.
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> They don't let you rewind or find out the size, but that's exactly why they can handle unbounded streams of data.
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If you've ever written a script that crashed with *out of memory*, `yield` is your new friend.
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β [Previous: Arrow functions](/articles/from-tg/series/php-can-do/arrow-functions) Β· [Whole series](/articles/from-tg/series/php-can-do) Β· [Next: DateTimeImmutable β](/articles/from-tg/series/php-can-do/datetime-immutable)
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---
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*First published in Ukrainian in the Telegram channel [βCreated to dieβ](https://t.me/php_while_true/78) on October 18, 2025.*