$appVersion = "5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.133 Safari/537.36";

if( preg_match( '/Chrome\/(?P<Version>[0-9.]{1,})/', $appVersion, $result ) )
{
    echo $result[2]; // Ergebnis: "57.0.2987.133"
}