strlen() / mb_strlen() Since: PHP 4（2000）

// Returns the number of bytes.
strlen($string);

// Returns the number of characters. Supports multibyte characters.
mb_strlen($string, $encoding);

strlen() returns the number of bytes in a string as an integer. mb_strlen() returns the number of characters as an integer.

<?php
// Get the length of an ASCII string.
$str = "Hello";
echo strlen($str); // Outputs '5'.

// Get the byte count of a string containing multibyte characters.
$name = "Taro Yamada";
echo strlen($name); // Outputs '11'.

// Use mb_strlen() to get the correct character count.
echo mb_strlen($name, "UTF-8"); // Outputs '11'.

// Useful for validating user input length.
$username = "John Smith";
if (mb_strlen($username, "UTF-8") > 20) {
	echo "Username must be 20 characters or fewer.";
} else {
	echo "Username accepted."; // This line is output.
}

// The length of an empty string is 0.
echo strlen(""); // Outputs '0'.

strlen() returns the length of a string in bytes. ASCII characters are 1 byte each, but Japanese characters in UTF-8 are 3 bytes each, so strlen() will not return the correct character count for strings containing Japanese. Always use mb_strlen() when working with multibyte strings.

mb_strlen() is the multibyte-aware version of the function and returns the correct character count based on the specified encoding. The second argument specifies the encoding; if omitted, the encoding set by mb_internal_encoding() is used.

This function is commonly used to validate form input length or to check a string's length before extracting a substring. To search within a string, use strpos(). To extract a substring, use substr() / mb_substr().