PHP executable not found. Install PHP 7 and add it to your PATH or set the php.executablePath setting

You installed PHP IntelliSense extension, and this error because of it.
So if you want to fix this problem go to this menu:
File -> Preferences -> Settings
Now you can see 2 window. In the right window add below codes:

{
    "php.validate.executablePath": "C:\\wamp64\\bin\\php\\php7.0.4\\php.exe",
    "php.executablePath": "C:\\wamp64\\bin\\php\\php7.0.4\\php.exe"
}

Just like below image.

NOTICE: This address C:\\wamp64\\bin\\php\\php7.0.4\\php.exe is my php7.exe file address. Replace this address with own php7.exe.