What does PHP do with the emoji characters on the mobile side?

How does PHP handle the emoji characters submitted by mobile?
Now the mobile side provides a lot of expressions, such as Baidu Input method in the expression.
A lot of expressions in PHP, after it is not recognized content, when stored in MySQL, it will start from the expression position, the contents of the back will be lost.

such as user input: Start content [Emoticons] End content
Get it in PHP and save it in MySQL only: start content [


Is there any way to remove these unrecognized expressions without affecting the content behind them?
For example, if the expression above is not recognized, it also wants to be able to be stored in MySQL: start content [] End content

------to solve the idea----------------------
Required to set the MySQL character set to Utf8mb4

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