See releases to download a JAR file. Almost certaintly you want the php-password-hash-VERSION-jar-with-dependencies.jar. This JAR file should be uploaded following the
normal procedures
for a custom extension.
If you need to rebuild the JAR file for use with a more modern version of Java, simply mvn clean install will generate the required artifacts.
Tested with Keycloak 15-22, but generally speaking it should work on most (all) versions of Keycloak.