In case this might help someone, after installing the thread safe version of PHP 5.5.1, everything was working under apache for my dev sites, but I ran into the same "php5.dll is missing" problem installing Composer using the Composer-Setup.exe - or, as I soon discovered, just running something as simple as php -v from the command line. I made a copy of php5ts.dll and named it php5.dll and everything worked. I assume the Composer installer was specifically looking for "php5.dll" and I knew that the thread safe code would be run by the renamed .dll. I also assume something is wrong with my setup to screw up the command line functionality, but with everything working, I have more important issues to deal with than to try and find the problem.