I had the same problem with OE Windows Server 2003 ... I had to downgrade to previous release

Which extension(s) are enabled in the php.ini?

It could be due to some extensions requiring external DLLs (oracle for example) or the PHP directory not being in your PATH.

Try to call php from the command line as well, with or without using the system php.ini (be sure that you have removed or updated the 5.2's php.ini, especially the extension_dir setting).

php.exe -n -i no php.ini loaded
php.exe -i with system php.ini

pajoye asked me which extensions were enabled.  I'm sorry but because I have uninstalled 5.3.1 and installed 5.2.13 I can no longer say for certain.  I do not have a copy of the php.ini from the 5.3.1 installation.  I checked my trash, and it's gone.

When I installed 5.3.1 I did go through the custom setup and to select extensions.  There were a few extensions that I knew I needed.  I think that they (PEAR and MySQL) were both selected by default, so I just took the defaults.

This were the installed extension, but now I have gone back to 5.2.12 because it is a production server

extension=php_dbase.dll
extension=php_gd2.dll
extension=php_imap.dll
extension=php_interbase.dll
extension=php_mysql.dll
extension=php_openssl.dll
extension=php_pdo_mysql.dll
extension=php_xsl.dll
extension=php_zip.dll

interbase is not available in 5.3, so I think you were using the old php.ini or messing extensions.

I just had the same problem as randy and adding the path where php resides to my PATH varible did the trick indeed!

@cunobatis at bluewin dot ch

that's a configuration problem. Adding the PHP directory to your PATH is a required step when you configure PHP with Apache.

Two points:
1. I did not have to manually define any PATH variables when I installed PHP 5.2.13 after I had uninstalled 5.3.1.  So the 5.2.13 installer is working where the 5.3.1 failed.
2. After I got PHP 5.2.13 running I installed PEAR.  I ran into a non-fatal installation problem in PEAR as well.  I found out that Windows 7 is case sensitive when referring to PATHs.  PEAR was creating it's PATH variable with the directory in all upper, but the actual folder was all lower.  That worked on earlier Windows, but not on 7.  I don't know which Windows version they made that change in.  But once I edited PEAR's PATH variables to match the actual directory case the non-fatal error was resolved.

I suspect that PHP 5.3.1's installer is creating a path with the wrong case, based upon the assumption that Windows doesn't care.  Can't prove it without uninstalling PHP, and I can't do that right now.

It's not the path variable; it's the dll file. I had exactly the same issue. I made sure my path had the right case and tried starting Apache: no dice. I had backed up my old php (from PHP 5.2.11) folder before installing PHP 5.3, so I tried copying php5apache2_2.dll from the backed up folder into the installation folder for PHP 5.3. Bingo! Apache started up and seems to be running fine. (Haven't tested and PHP 5.3 only features yet, though.)

@smacdav at gmail dot com

Your install is a mess then. php 5.2's sapi dll cannot work nor load with 5.3.

Please ask php-setup or php-windows mailing for further supports (to any other reporters looking for help regarding the problem described here).

You're right, of course. I was just realizing the issue and coming back to comment. That was silly of me. Sorry.

I had the same problem with 5.3.5 on Windows 7 with Apache 2.2. Here's how I fixed 
it. When reinstalling Apache (as it failed to start after installing PHP) I got 
the error 'failed to load php5apache2_2.dll'. That of course was a 
clue. Checking httpd.conf I saw that the PHP installer had not set the PHP ini 
directory correctly (just empty quotes) and the path to the DLL was not added. I 
manually fixed these (I installed PHP to C:\PHP) and everything worked fine.

By the way, based on comments above and elsewhere, I checked the path was set with 
correct case. Everything was in order so that wasn't the problem in this case. 
Hope this helps.

I also ran into a similar error while trying to install PHP 5.2.17 on Windows XP.  Apache would crash after trying to restart it.

To fix Apache from crashing (and after reading pajoye's comment on some extensions requiring external DLLs), I uninstalled/removed PHP (using the PHP windows setup program), then re-installed PHP, but this time selecting only a handful of extensions to be installed (i.e. curl, dba, mbstring, mcrypt, mhash, mysql, mysqli, openssl) as opposed to opting for all extensions to be installed, like I did the first time.  

After making appropriate changes to Apache's config file "httpd.conf"

 PHPIniDir "C:/Program Files/PHP/"
 LoadModule php5_module "C:/Program Files/PHP/php5apache2_2.dll"

and verifying system variables and Path were in order, I restarted Apache and this time it worked!