PHP strlen Function: How It Works with Strings

Developers needed a way to check the length of a string, so the strlen function was introduced to handle this task in PHP.

Understand the strlen() Function in PHP

The strlen() function in PHP tells you how many characters a string has. It counts each byte, not each letter.

Here is the syntax:

strlen( $string )

You pass one string and it gives you a number back. It gives you an integer. That number shows how many bytes the string holds.

PHP counts every byte in the string. It starts from the first character. It stops when it reaches the end.

Here is an example:

echo strlen("Flatcoding");

This prints 5. That string has five letters, and each letter uses one byte.

echo strlen("");

This gives 0. The string has no content, so the count is zero.

Here are the use cases:

• You check if a field is empty and check if it is too short.
• It stops weak passwords and makes sure they are not shorter than a safe length.
• You can cut long strings and check their length first with strlen().

Handle Multibyte Strings (Important Warning)

strlen() does not count letters in multibyte strings. It counts bytes. If a letter uses more than one byte, strlen() gives the wrong length.

To handle this, use mb_strlen().

echo strlen("你好");
echo mb_strlen("你好");

The output:

6
2

Unicode has characters that use two or more bytes. strlen() does not treat them as one unit. That causes problems with length checks or output limits.

The Difference Between strlen() an mb_strlen()

The strlen() counts the number of bytes in a string.

It works well for ASCII strings. Each character takes one byte. But for multibyte characters, like those in UTF-8 (e.g. Arabic, Chinese, or emoji), strlen() counts each byte, not each character.

For example:

$str = "こんにちは";
echo strlen($str); 

Output:

Hello
15

This string has 5 characters, but strlen() returns 15 because each character uses 3 bytes in UTF-8.

While the mb_strlen() counts the number of characters, not bytes.

It works correctly for multibyte strings. You must set the correct encoding. Most of the time, that is UTF-8.

$str = "こんにちは";
echo mb_strlen($str, "UTF-8"); // Output: 5

Now the function shows the correct count: 5 characters.

Examples

Simple check:

$username = "admin";
if (strlen($username) < 4) {
    echo "Username too short";
}

This checks if the username has fewer than four characters. It runs fast and works for basic cases.

Multibyte problem:

$name = "José";
echo strlen($name);      
echo mb_strlen($name);  

strlen($name) returns 5 because strlen counts bytes, and the character é (accented e) is a 2-byte character in UTF-8. So "José" consists of:

• J → 1 byte
• o → 1 byte
• s → 1 byte
• é → 2 bytes

mb_strlen($name) returns 4 because mb_strlen counts characters, not bytes. It correctly handles multibyte characters like é.

Wrapping Up

In this article, you learned how strlen() counts the bytes in a string. You also saw why it fails with multibyte characters.

Here is a quick recap:

• Use strlen() for basic strings
• Use mb_strlen() for multibyte or Unicode strings
• strlen() counts bytes, not real characters
• It helps with form checks and limits with trims

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