Can someone tell me how to query in the SEDE to get the related tags of php ?
You could use this comically inefficient query I just created.
It just looks for all of the Posts that have the tag you're searching for, then groups by the other tags on those posts, then displays those other tags in descending order by quantity.
It appears to line up precisely with the sidebar on the tags I tried. Here's php:
Note: User input is case-sensitive on that query, because I'm lazy =)