Will php mb_strlen($str,‘utf8’) ever return a greater result than JavaScript .length?

I’m working on an Angular 17 reactive form where I send the form data to a PHP API on the server and store it in a database.
I would like the user to be able to input emojis to the form so I have set my database to utf8mb4_unicode_ci collation so that the emojis can be stored.

Security is very important to me so I do several checks on both the client side and the server side for various things.

One of the checks I do is to check the length of the input. I was wondering If you can help because the length results are inconsistent on client side and server side (since the string contains emojis).

On using the JavaScript .length property and also the built-in Angular Form Validators called minLength and maxLength I see that they all calculate the length in the same way (for example most of the basic smilie emojis are calculated as having a length of 2).

However when I send this data (which includes emojis) to the server side I use the PHP method called mb_strlen($subject, 'utf8') and the values are different (most of the basic smilie emojis are calculated as having a length of 1 and also they take up 1 varchar character in the database).

I’ve tested about 160 emojis to see what values they return on both client side and server side in order to try and work out a pattern (so that I can do checks for length in the right way).

As you can see from my screenshots below in most cases mb_strlen($subject,‘utf8’) returns a lower value for the length than JavaScript .length. Sometimes it returns the same value as JavaScript .length property but in all these cases mb_strlen($subject,‘utf8’) has never returned a length greater than what JavaScript .length returns.

Is it safe to assume that mb_strlen($subject,‘utf8’) will never return a value greater than JavaScript .length. for the rest of existing emojis that I have not tested?

If not could you explain a bit more about this and could you give some examples of characters where mb_strlen($subject,‘utf8’) would return a greater value than JavaScript .length?

Thank you

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