Byte

A byte is a unit of digital information storage that typically consists of 8 bits. It is the fundamental building block for representing data in computers. A byte can store values ranging from 0 to 255 (in an unsigned 8-bit representation).

PHP uses bytes when manipulating strings. Bytes are characters, as long as the string uses ASCII representation, which is by default. Characters might be larger than a byte, and they become multi-bytes.

<?php

    $string = abcd;

    echo strlen($string); // 4

?>

Related : Character, Multi-byte, American Standard Code for Information Interchange (ASCII)