PHP Manual: SplFileObject::fwrite

public function SplFileObject::fwrite(string $data, ?int $length = null): int|false

Writes the contents of data to the file

data: The string to be written to the file.
length: If the length argument is int, writing will stop after length bytes have been written or the end of data is reached, whichever comes first.

Returns the number of bytes written, or false on error.

Version	Description
8.5.0	`length` is now nullable.
7.4.0	The function now returns `false` instead of zero on failure.

Example #1 SplFileObject::fwrite() example

<?php
$file = new SplFileObject("fwrite.txt", "w");
$written = $file->fwrite("12345");
echo "Wrote $written bytes to file";
?>

The above example will output something similar to:

Wrote 5 bytes to file

SplFileObject::fwrite