On the side bar of the PHP 7 alpha download page, it does say this:
VC9, VC11 & VC14 More recent versions of PHP are built with VC9, VC11 or VC14 (Visual Studio 2008, 2012 or 2015 compiler respectively) and include improvements in performance and stability.
The VC9 builds require you to have the Visual C++ Redistributable for Visual Studio 2008 SP1 x86 or x64 installed
The VC11 builds require to have the Visual C++ Redistributable for Visual Studio 2012 x86 or x64 installed
The VC14 builds require to have the Visual C++ Redistributable for Visual Studio 2015 x86 or x64 installed
There's been a problem with some of those links, so the files are also available from Softpedia.
In the case of the PHP 7 alpha, it's the last option that's required.
I think that the placement of this information is poor, as it's kind of marginalized (i.e.: it's basically literally in the margin!) whereas it's actually critical for the software to run.
I documented my experiences of getting PHP 7 alpha up and running on Windows 8.1 in PHP: getting PHP7 alpha running on Windows 8.1, and it covers some more symptoms that might crop up. They're out of scope for this question but might help other people.
Other symptom of this issue:
php7apache2_4.dll is missing despite it definitely being in place, and offering nothing else in any log.php-cgi.exe - The FastCGI process exited unexpectedly (as per @ftexperts's comment below)Attempted solution:
php7apache2_4.dll file from an earlier PHP 7 dev build. This did not work.(I include those for googleability.)