use alcamo\iterator\InputStreamLineIterator;

$iterator = new InputStreamLineIterator(fopen(__FILE__, 'r'));

foreach ($iterator as $line => $data) {
    printf("%3d %s\n", $line, $data);
}

This will output its own source file with line numbers.

Provided classes and traits

IteratorCurrentTrait provides a partial implementation of the Iterator interface so that a class using this trait just needs to implement the methods rewind() and next().
InputStreamLineIterator is an iterator class that reads lines from an input stream. Flags can be given e.g. to skip empty lines. Derived classes can easily add other features such as skipping comment lines.