SPX, which stands for Simple Profiling eXtension, is just another profiling extension for PHP. It differentiates itself from other similar extensions as being:

totally free and confined to your infrastructure (i.e. no data leaks to a SaaS).
very simple to use: just set an environment variable (command line) or switch on a radio button (web request) to profile your script. Thus, you are free of:
manually instrumenting your code (Ctrl-C a long running command line script is even supported).
using a dedicated browser extension or command line launcher.
multi metrics capable: 22 are currently supported (various time & memory metrics, included files, objects in use, I/O...).
able to collect data without losing context. For example Xhprof (and potentially its forks) aggregates data per caller / callee pairs, which implies the loss of the full call stack and forbids timeline or Flamegraph based analysis.
shipped with its web UI which allows to:
enable / configure profiling for the current browser session
list profiled script reports
select a report for in-depth analysis, featuring these interactive visualizations:
timeline (scale to millions of function calls)
flat profile
Flamegraph