Actually, this method can be useful to test a generator before iterating, as it executes your function up to the first yield statement. I.e. if you try to read a non-existent file in a generator, an error will normally occur only in client code foreach()'s first iteration. Sometimes this can be critical to check beforehand.
Take a look at a modified example from here:
http://php.net/manual/ru/language.generators.overview.php#112985
<?php
function getLines($file) {
$f = fopen($file, 'r');
try {
while ($line = fgets($f)) {
yield $line;
}
} finally {
fclose($f);
}
}
$getLines = getLines('no_such_file.txt');
$getLines->rewind(); // with ->rewind(), a file read error will be thrown here and a log file will not be cleared
openAndClearLogFile();
foreach ($getLines as $n => $line) { // without ->rewind(), the script will die here and your log file will be cleared
writeToLogFile('reading: ' . $line . "\n");
}
closeLogFile();
?>
P.S.: When you iterate over a generator after ->rewind(), you'll get the first yielded value immediately, as the preceding code was already executed.