Concatenation .
Concatenation is the PHP feature that appends two strings together, to make a third, longer one.
Concatenation is also achieved with interpolation, inside a string; by calling echo, multiple times or with multiple arguments; by using the join() function.
Concatenation is done with the dot operator ..
<?php
$a = "world";
$b = "Hello ";
echo $b . $a;
// displays Hello world
?>