Question: The test codes are written using SimpleTest framework, and I am looking for a tool or an open source solution to perform Code Coverage.

Answer: 
Use SimpleTest's Coverage extension:

<https://github.com/simpletest/simpletest/tree/master/extensions/coverage>
You can use **phpunit** 

Read about it, mostly used. 

<http://phpunit.de> -- page is in english 

Git: <https://github.com/sebastianbergmann/phpunit>

CodeCoverage: <http://phpunit.de/manual/current/en/code-coverage-analysis.html>