Using PHP mb_substr() for Multibyte String Truncation

The mb_substr() function is a PHP string‑truncation utility that works on multibyte strings, correctly handling characters like Chinese or Japanese, unlike the regular substr() which may produce garbled results.

mb_substr() Function Signature

<code>string mb_substr ( string $str , int $start [, int $length [, string $encoding ]] )</code>

Where $str is the input string, $start is the zero‑based start position, $length (optional) is the number of characters to extract, and $encoding (optional) specifies the character encoding, defaulting to the script’s encoding (usually UTF‑8).

Usage Examples

Example 1

<code>$str = "今天是个好日子";
echo mb_substr($str, 0, 2); // outputs "今天"
</code>

Because each Chinese character occupies two bytes, extracting the first two characters yields "今天".

Example 2

<code>$str = "abcdefg你好";
echo mb_substr($str, 1, 4); // outputs "bcde"
</code>

Starting from the second character and taking four characters returns "bcde"; the multibyte characters are handled correctly.

Example 3

<code>$str = "abcdefg你好";
echo mb_substr($str, 4); // outputs "g你好"
</code>

If the third parameter ( $length ) is omitted, the function extracts to the end of the string.

Note that if $start exceeds the string length, mb_substr() returns an empty string, and a negative $length is ignored, causing extraction up to the string’s end.

When building multilingual websites, mb_substr() is especially useful because it guarantees correct handling of multibyte characters, preserving page layout and readability.

Overall, mb_substr() is a practical PHP function for safely truncating multibyte strings, essential for developers working on internationalized or multilingual projects.