Assignment operators is used for writing values to the variables. Here the first operand must be a variable. The value of assignment operator is the final value assigned to the variable.
$value = 15
The above expression has the value 15 assigns 5 to $value.
Several assignment operators are composites of an operator followed by an equal sign.
These composite operators apply the operator taking the variable on the left as the first operand and the value on the right as the second operand, and assign the result of the operation to the variable on the left.
$counter += 7; // This is identical to $counter = $counter + 7;