There are two string operators. The first is the
concatenation operator ('.'), which returns the concatenation of its
right and left arguments. The second is the concatenating assignment
operator ('.='), which appends the argument on the right side to
the argument on the left side. Please read Assignment
Operators for more information.
<?php
$a = "Hello ";
$b = $a . "World!"; // now $b contains "Hello World!"
$a = "Hello ";
$a .= "World!"; // now $a contains "Hello World!"
?>
See also the manual sections on the String type and String functions.