“Cannot validate the php file. The php program was not found”

First, go to File->Preferences->User settings,
A window will come like below image.

Now change the setting.json file like below image and save:

If PHP is not installed in your system, download php from here: windows.php.net/download, unzip to C: drive, set "php.validate.executablePath": "C://path.to.your.php.folder//php.exe"

If php is installed in your system simply set the path of your php.exe file like below:

"php.validate.executablePath": "C://path.to.your.php.folder//php.exe"

hint: In my case I use XAMPP which is installed in the D: drive, so my setting is below

"php.validate.executablePath": "D://xampp//php//php.exe"

Mind the double forward slashes.