This library builds a radix tree and outputs it as a php code, as well as a regular expression. Here is a post explaining the source code. It covers fundamentals of Radix trees. Also, it visualizes an algorithm for inserting a new leaf node, as well as an algorithm for looking up a leaf node.

Run tests with

php ../vendor/phpunit/phpunit/phpunit --exclude-group slow ./

If you want to run slow tests, run them one by one since they are very demanding memory-wise.