Concatenation
Definition: Concatenation occurs when you glue two or more bits of data together in a chain to create a new value:
$myVariableOne = "some data"; $myVariableTwo = "and more data"; print $myVariableOne . " " . $myVariableTwo;
This outputs "some data and more data". The concatenation operator in this case is the period, and acts as the glue.
Source...