Dtrace. however, people involved in building and debugging applications that run under Linux or Solaris and people who are responsible for tuning and optimizing performance of mission critical systems will most likely be using Dtrace or at least they should start understanding it and possibly start using it. Dtrace is a dynamic tracing framework developed by Sun Microsystems for troubleshooting kernel and application issues in real time.
Dtrace gives you the option to truley see what your application is doing and how your operating system kernel is handling stuff. A real view of what is happening can be very good when you are diving deep into debugging applications and issues with your servers.
In the past Dtrace has primarily be used for "server" based applications and not si much for scripting languages. A not well know fact is that Dtrace is also a part of the PHP language. David Soria Parra has merged the Dtrace functions into PHP from PHP version 5.4 and onwards. The original way of using PECL to incorporate Dtrace into PHP.
If you are serious about building PHP applications and serious about finding bottlenecks in your code and the way your PHP code is interacting with your kernel you should look into Dtrace for PHP. Chritoper Jones wrote an article back in 2012 named "Adding DTrace Probes to PHP extensions" and recently wrote an article named "DTrace with PHP update" which are both good starting points. You can find both on the Oracle website in the PHP and Oracle blog from Christopher Jones.