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

{
...
"php.validate.executablePath": "/opt/lampp/bin/php",
...
}

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