Optimizing PHP string concatenation

$foo = 'John SMITH';

echo "Hello $foo, welcome on my website.";

echo "Hello " . $foo . " welcome on my website.";

echo 'Hello ' . $foo . ' welcome on my website.';

echo 'Hello ', $foo , ' welcome on my website.';

0.0022029876709
0.00211095809937
0.00213599205017
0.00551700592041