How to find out which part of PHP code loads CPU on the server?

Google "PHP profiling"
XDebug, XHprof, Blackfire ...

As well as "load testing"
jmeter ...

For a quick test short scripts: take measurements, cut out a piece of repeated measurements.

Typical places with problems:
session based access to the file system (default);
- sub-optimal calls to the database;
- long cycles (often with calls to the database).

Look for such places in your script, and then as advised Yan-s : take measurements, cut out a piece of repeated measurements.

1) Install the php extension xdebug
Enable this extension and profiling in xdebug settings
xdebug.profiler_enable = 1
Please note these 2 options (where to put profile files):
xdebug.profiler_output_dir
xdebug.profiler_output_name
2) Deflate the profiling I've found myself on a development machine, then feed it to this utility as QCacheGrind. It will quickly analyze them and give you what methods, how many times have you volunteered, and how much CPU time left for processing. You can sort the results.
3) xdebug In production better off.

offer supersymetry option.
in the beginning of the script place $start = microtime(true);
and code scattering printf(microtime(true) - $start);