What does the .= operator mean in PHP?

This is for concatenation

$var  = "test";
$var .= "value";

echo $var; // this will give you testvalue

the "." operator is the string concatenation operator. and ".=" will concatenate strings.

Example:

$var = 1;
$var .= 20;

This is same as:

$var = 1 . 20;

the ".=" operator is a string operator, it first converts the values to strings; and since "." means concatenate / append, the result is the string "120".

It's the concatenating assignment operator. It works similarly to:

$var = $var . "value";

$x .= differs from $x = $x . in that the former is in-place, but the latter re-assigns $x.