MySQL User Defined Function

Hello again. I ask you, dear community, for help.

Many times I heard that in MySQL you can write your own functions (in order to get around large bulkheads in PHP). I read the manuals, and did not understand. You can’t write your own function without using third-party resources? Those. in any case, you have to either compile with your source ... or connect the library?

Could you tell in more detail, who faced this? And how much more effective it can be than a PHP overkill.

PS .: I’m asking a lot of questions of a different nature, because at work to understand the manual, unfortunately there is no time. Therefore, I ask for help.

PPS: On php, I have a recursive function that goes along the branch of goods (id: 145 -> (goods) -> id: 156 (child 145th) -> goods-> etc.)

You need to make a selection, so I think that in PHP it will not be advisable.