Why can't I use keyboard emojis on forums?

[Elfilin]

Everytime I use an emoji from my keyboard on forums, it gives me an error like this:

SQL ERROR [ mysqli ]

Incorrect string value: '\xF0\x9F\x98\x95' for column 'post_text' at row 1 [1366]

An SQL error occurred while fetching this page. Please contact the Board Administrator if this problem persists.

I am on mobile at the time of writing this.

[United Calanworie]

Because you can’t. The forum software doesn’t support it. You’re restricted to the smilies.

[Trotterdam]

To be precise, you can't use Unicode characters higher than U+FFFF. This includes emojis, but also the alphabets of some obscure languages, and other such stuff that people just don't use very often.