The most common reason for this is that the default php.ini for current versions of PHP does not enable the mysql interface dll (what were they thinking.) Find the following line in the php.ini and remove the ; to uncomment it -
change this -
to this -
Under IIS, you must reboot to get this change to take effect.
Fatal error: Call to undefined function mysql_connect() in c:\Inetpub\wwwroot\PHP\connmysql.php on line 5
PHP Warning: PHP Startup: Unable to load dynamic library './php_mysql.dll' - The specified module could not be found. in Unknown on line 0
This sounds like an incomplete installation of PHP.
Which windows binary did you download and install? If you got the .zip full version, it includes the extension dll's. If you got the .exe installer version, it does not include any extensions.
There is also an extension directory name in the php.ini that must be set. Find the following line in your php.ini and make sure that the path in the quotes points to the location where the extension .dll's are located -