The php command cannot be found. Please verify that PHP is installed, or set the php.executables setting.

Open settings.json and put this code below for xampp users  
"php.validate.executablePath": "C:\\xampp\\php\\php.exe",
  "php.executablePath": "C:\\xampp\\php\\php.exe"

Open Settings.json and put this codes below: for wamp users
"php.validate.executablePath": "C:\\wamp64\\bin\\php\\php7.1.9\\php.exe",
  "php.executablePath": "C:\\wamp64\\bin\\php\\php7.1.9\\php.exe"