Troubleshooting xdebug profiler

Xdebug can be used to profile your PHP applications. However, it can be difficult to configure and get working. There are a number of gotchas to look out for.


To enable the profiler on every request you can use the setting:

xdebug.profiler_enable = 1

If you want to just enable it for certain requests you can use:


This will allow you to use ?XDEBUG_PROFILE=1 in your query string. However, if you are using mod_rewrite to rewrite your URLs in Apache, you need to make sure the rule has the [QSA] flag in it to pass the query string through or this will not work.

Output direcotry

You can set the output directory to whatever directory you wish using the output dir setting:

xdebug.profiler_output_dir = '/tmp/xdebug'

However, xdebug will not create this directory for you. You need to manually create the directory and give write permissions to the web server too.

Profiling SQL queries with EXPLAIN

If you have a complex SQL query, you might find that performance isn’t exactly ideal. Worse still, you don’t actually know which part of the query it is that is actually taking so long.

Luckily, MySQL comes to the rescue with the ability to explain.

All you have to do is start your query with the keyword EXPLAIN and MySQL will, rather than returning you a recordset of results, will instead provide a break down of everything it has done, including how it made the table joins and what order it did everything in.

EXPLAIN SELECT a INNER JOIN b ON a.col1 = b.col2 WHERE a.col1 > 1 AND b.col2 > 2;