SPL Iterators In Depth

Also Known As

TL;DR

Explanation

SPL iterators implement the Iterator interface for lazy traversal — only loading data when requested. Key iterators: SplFileObject (iterate file lines without loading all into memory), DirectoryIterator / RecursiveDirectoryIterator (filesystem traversal), RecursiveIteratorIterator (flatten recursive iterators), FilterIterator (filter elements lazily), LimitIterator (slice an iterator), CachingIterator (look-ahead). They all work with foreach and can be chained. Critical for processing large files (CSV imports, log files) where file_get_contents() would exhaust memory.

Common Misconception

Why It Matters

Common Mistakes

• file() or file_get_contents() for large files — use SplFileObject instead.
• Not using RecursiveIteratorIterator to flatten RecursiveDirectoryIterator — it returns recursive structure.
• Creating custom iterators without implementing valid() — foreach relies on valid() to stop.
• Not rewinding an SplFileObject before re-iterating — it tracks position persistently.

Code Examples

// Loads entire 2GB file into memory:
$lines = file('/var/log/huge.log'); // Fatal: memory exhausted
foreach ($lines as $line) {
    processLine($line);
}

// SplFileObject — O(1) memory regardless of file size:
$file = new SplFileObject('/var/log/huge.log');
$file->setFlags(SplFileObject::READ_AHEAD | SplFileObject::SKIP_EMPTY);
foreach ($file as $line) {
    processLine($line); // Only current line in memory
}

// FilterIterator — lazy filter without loading all:
class ErrorLineIterator extends FilterIterator {
    public function accept(): bool {
        return str_contains($this->current(), 'ERROR');
    }
}
$errors = new ErrorLineIterator(new SplFileObject('/var/log/app.log'));
foreach ($errors as $errorLine) { /* only ERROR lines */ }

References

• ↗ https://www.php.net/manual/en/spl.iterators.php

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