I\'m programming a script using PHP and MySQL and I want to get a
unique id (consisting of a string: capitals and small
letters with numbers) like: gHYtUUi5b.
You could use Twitter's snowflake.
In short, it generates a unique id based on time, server id and a sequence. It generates a 64-bit value so it is pretty small and it fits in an INT64. It also allows for sorting values correctly.
https://developer.twitter.com/en/docs/basics/twitter-ids
In sum, it allows multiple servers, highly concurrency, sorting value and all of them in 64 bits.
Here it is the implementation for MySQL
https://github.com/EFTEC/snowflake-mysql
It consists of a function and a table.