In a typical PHP installation, the need for high performance almost always results in optimization at the cost of debugging facilities. This is a reasonable tradeoff for production use, but when developing an extension it falls short. What we need is a build of PHP which will give us some hints what has gone wrong when something does.
The Zend Engine provides a memory manager which is capable of tracking memory leaks in extensions and providing detailed debugging information. This tracking is disabled by default, as is thread-safety. To turn them on, pass the --enable-debug and --enable-maintainer-zts options to configure, along with whatever options you typically use. For instructions on building PHP from source, see the instructions at Generelle Überlegungen. A typical configure line might look like this:
$ ./configure --prefix=/where/to/install/php --enable-debug --enable-maintainer-zts --enable-cgi --enable-cli --with-mysql=/path/to/mysql