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The %= (modulo assignment) operator is a combined arithmetic-assignment operator that divides the variable on the left operand by the expression on the right operand, and assigns the integer remainder of that division back to the left operand.

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Syntax

$variable %= expression;
This is functionally equivalent to the expanded assignment form: 
$variable = $variable % expression;

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Technical Mechanics

  • Implicit Integer Casting: Before the modulo operation is executed, PHP strips the fractional parts of both operands, implicitly coercing them into integers. For example, 5.9 %= 2.8 is evaluated strictly as 5 % 2.
  • Signage: The sign of the resulting remainder is determined exclusively by the dividend (the left operand). The sign of the divisor (the right operand) is ignored during evaluation.
  • Floating-Point Limitation: Because the %= operator forces integer coercion, it cannot be used to find the remainder of floating-point divisions. Developers must use the fmod() function for floating-point modulo operations.
  • Exceptions: If the right operand evaluates to 0, PHP 8.0 and later will throw a DivisionByZeroError exception.

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Evaluation Examples

// Standard integer evaluation
$a = 10;
$a %= 3; 
// Evaluates as: 10 % 3. $a becomes 1.

// Implicit float-to-integer coercion
$b = 10.8;
$b %= 3.9; 
// Evaluates as: 10 % 3. $b becomes 1.

// Negative dividend (result takes the sign of the left operand)
$c = -10;
$c %= 3; 
// Evaluates as: -10 % 3. $c becomes -1.

// Negative divisor (sign of the right operand is ignored)
$d = 10;
$d %= -3; 
// Evaluates as: 10 % -3. $d becomes 1.
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