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The -= operator is a compound assignment operator in PHP that subtracts the value of the right operand from the left operand and assigns the resulting value back to the left operand.

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Syntax

$variable -= expression;
While functionally equivalent to the expanded assignment form ($variable = $variable - expression;), the -= operator evaluates the left-hand side exactly once. If the left operand is an expression with side effects—such as an array index determined by a function call—the compound assignment executes the function only once. In contrast, the expanded form would evaluate the function twice.

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Technical Characteristics

  • L-value Requirement: The left operand must be a valid variable reference (an l-value) capable of receiving an assignment.
  • Type Coercion and Promotion: The operator expects numeric operands (integers or floats).
    • If an integer and a float are used, the integer is implicitly promoted to a float during the arithmetic operation, resulting in a float.
    • If a numeric string is provided, PHP performs implicit type juggling to convert the string to an integer or float before subtraction.
    • In PHP 8.0 and later, passing a non-numeric string or incompatible type will throw a TypeError.
  • Return Value: Like all assignment operators in PHP, -= evaluates to the newly assigned value. This allows the operation to be embedded within larger expressions or chained.

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Mechanical Examples

// Standard integer subtraction assignment
$a = 10;
$a -= 4; 
// $a is now 6 (int)

// Implicit float promotion
$b = 5;
$b -= 2.5; 
// $b is now 2.5 (float)

// Right operand expression evaluation
$c = 20;
$c -= (2 * 3); 
// $c is now 14 (int)

// Implicit type juggling (numeric string)
$d = 15;
$d -= "5"; 
// $d is now 10 (int)

// Return value evaluation
$e = 10;
$f = ($e -= 3); 
// $e is 7, $f is 7

// Single evaluation of the left-hand side
function get_index() {
    return 1;
}

$array = [10, 20, 30];
$array[get_index()] -= 5; 
// get_index() is called exactly once. $array[1] is now 15.
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