/* File->Preferences->settings->User settings tab->extensions->from the drop down select php->on the right pane under PHP › Validate: Executable Path select edit in settings.json.

Then set the path as your case may be e.g for a xamp user who installed xammp on c drive you will have:
*/
"php.validate.executablePath": "c:\\xampp\\php\\php.exe"

/*  If php is installed in your system independently or by other means simply set the path of your php.exe file like below:
*/
"php.validate.executablePath": "C://path.to.your.php.folder//php.exe"