Alternative approach

[SamMousa]

I got stuck and thought I'd go with another approach.

https://github.com/SAM-IT/cs2cs-php

This requires cs2cs to be installed on the machine running PHP but since I run my own servers for me that's not an issue.

[judgej]

There are other libraries that take this approach - basically hand over the conversions to a server-installed library. Some go via MySQL, which I believe also does this: you install the correct libraries and MySQL uses those for its GIS functionality. Going through MySQL means you don't have to worry so much about access to command-line tools, as MySQL handles all that - you just need a database connection.

[SamMousa]

Yes I looked into that.
Those extensions on MySQL require custom MySQL configuration so the effort is basically the same.
Anyway thought this might be of use to you in case you need it. I can no longer devote my time to trying to recreate proj4 in PHP since it doesn't add any value for me.

Good luck!