Concatenate string literals (string enclosed in double quotes).
"." concatenate strings including variables.
Additionally, numeric variables are also able to be concatenated.

<?php

$panda     = "panda";
$attribute = "is cute.";

// Concatenate string and variables using .
echo $panda . " " . $attribute . "\n";

      

$ php concatenate_string_literal.php
panda is cute.