The basic assignment operator is "equal to". It means that the left operand gets the value of the expression on the right.

    <?php
    $a = ($b = 4) + 5;
    ?>

Here the value of $a is equal to 9 now, and $b has been set to 4.

The assignment copies the original variable to the new one using the concept of assignment by value, so changes to one will not affect the other.

    <?php
    $a = 3;
    $a += 5;
    $b = "Hello ";
    $b .= "There!"; 
    ?>

In the above example the value of $a is 8 as if we used an expression $a=$a+5. The value of $b would be the string "Hello there".