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The **= operator is the exponentiation assignment operator in PHP. It raises the variable on the left operand to the power of the evaluated expression on the right operand, and subsequently assigns the computed result back to the left-side variable. 
$a **= $b;
This operation is semantically equivalent to the expanded assignment syntax: 
$a = $a ** $b;

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Technical Mechanics

Type Coercion and Resolution Before execution, PHP evaluates the right-hand expression. In PHP 8.0 and later, applying arithmetic operators to incompatible types (such as arrays, objects, or non-numeric strings) throws a TypeError. For valid numeric strings, PHP implicitly casts them to an int or float. The expression evaluates to, and the left operand is assigned, a specific data type based on the operands and the mathematical result:
  • int: Evaluates to an integer if both operands are integers, the right operand (exponent) is greater than or equal to zero, and the computed value does not exceed PHP_INT_MAX or fall below PHP_INT_MIN.
  • float: Evaluates to a float if either operand is a float, if an integer base is raised to a negative integer exponent (which produces a fraction), or if the resulting integer overflows or underflows the system’s integer bounds (PHP_INT_MAX and PHP_INT_MIN).
  • float (NAN): Evaluates to NAN (Not a Number) if a negative base is raised to a fractional exponent.
Precedence and Associativity As an assignment operator, **= has very low precedence. Consequently, the entire expression on the right-hand side is fully evaluated before the exponentiation and assignment operations are performed. Like all assignment operators in PHP, **= is right-associative. In chained assignments, the right-most operations are evaluated first. For example, the expression $a **= $b **= $c is evaluated as $a **= ($b **= $c).

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Syntax Visualization

// Standard integer exponentiation
$base = 2;
$base **= 3; 
// $base is now 8 (type: int)

// Float resolution via negative integer exponent
$base = 2;
$base **= -1; 
// $base is now 0.5 (type: float)

// Float resolution via fractional exponent
$base = 9;
$base **= 0.5; 
// $base is now 3.0 (type: float)

// Right-hand expression precedence
$base = 2;
$base **= 2 + 1; 
// Evaluates as $base **= 3. $base is now 8 (type: int).

// Right-associativity in chained assignments
$a = 2;
$b = 3;
$a **= $b **= 2; 
// Evaluates $b **= 2 first ($b becomes 9). 
// Then evaluates $a **= 9 ($a becomes 512).

// Negative base with fractional exponent
$base = -1;
$base **= 0.5; 
// $base is now NAN (type: float)
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