Using MQTT in PHP has been possible for a long time using the Simple Asynchronous Messaging (SAM) MQTT class. Unfortunately this is an imperfect solution due to unclear licensing, some slightly dubious design decisions and bugs.
Thankfully, Andrew Milstead has started creating an alternative implementation. It is MIT licensed and available on github. It’s very new, so if you have problems check back to see if there have been updates and then let Andrew know.