Download the latest version of PHP on the official website, why not php5isapi.dll this DLL file in the unpacked installation package?

Since PHP 5.3.1, PHP is no longer supported in ISAPI mode, so you cannot see the Php5isapi.dll file in the PHP directory above the PHP5.3.1 version.
So, IIS6 running PHP 5.3.1 or later, you will not be able to continue using ISAPI mode to run PHP, but instead use the more advanced fastcgi mode.
The IIS6 itself is not integrated fastcgi, so you need to download and install the configuration fastcgi to the IIS.net site to run a higher version.
Then there is, your IIS6 is unable to run PHP5.6.4 such a high version, because IIS6 must be win7 the following system, then from PHP5.4 above, will no longer support Win7 (NT6.0) The following Windows operating system. So IIS6 's largest supported PHP version is the PHP5.4 series.
That is, iis6+fastcgi+php5.4

Download the latest version of PHP on the official website, why not php5isapi.dll this DLL file in the unpacked installation package?