If you want to add a custom collation in mysql/mariaDB, for utf-8 charsets you can modify .../charsets/Index.xml and extend the charset with the LDML-Syntax:
<charset name="utf8">
...
<collation name="utf8_myown_ci" id="1234">
<rules>
<reset>\u0000</reset>
<i>\u0020</i> <!-- space -->
...
</rules>
</collation>
...
</charset>
But there is not charset-tag with name "utf8mb4". So I created one with name="utf8mb4" and added collation/rules tags and in phpmyadmin i could choose the newly created collation. But i couldn't inserts four byte characters; i get the error
"#1366 - Incorrect string value: '\xF0\x9F\x8D\xB5\xF0\x9F...' for field ..."
(with the build in mb4-collation i can do it).
To be more precise: I have one column (a) with the bulit-in collation utf8mb4_general_ci and one column (b) with my own collation utf8mb4_myown_ci(defined in Index.xml). I insert the same data in both columns and in column a there is no error and in column b i'll get the error as described above.
I created the following entry in Index.xml:
<charset name="utf8mb4">
<family>Unicode</family>
<description>UTF-8 MB4 Unicode</description>
<collation name="utf8mb4_general_ci" id="45">
<flag>primary</flag>
<flag>compiled</flag>
</collation>
<collation name="utf8mb4_bin" id="46">
<flag>binary</flag>
<flag>compiled</flag>
</collation>
<collation name="utf8mb4_myown_ci" id="213">
</collation>
</charset>
It seems to be no problem to have the collation-tag empty, because i created an empty utf8_myown_ci inside charset="utf-8" and this works.
In the column with utf8mb4_myown_ci i can also insert 3 Byte Chars, so it seems it is interpreted as an utf8 collation.
I tried google multiple times and didn't find anything here, but i couldn't find any hints, how to add collations to charsets, which aren't present in Index.xml.
Any Ideas how to do it? Thank you for any hints!
Turns out, i used an occupied collation-ID. If i use e.g. 501 instead of 213, it works.