Null Coalescing Assignment

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Syntax

$leftOperand ??= $rightOperand;

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Logical Equivalence

// Using the null coalescing assignment operator
$a ??= $b;

// Is functionally equivalent to the following control structure:
if (!isset($a)) {
    $a = $b;
}

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Technical Characteristics

• Single Evaluation: The operator evaluates its left operand exactly once. If the left operand is a complex expression (such as a function returning a reference, or a property access), it is not evaluated twice (once to read, once to write).
• Assignment Bypass: If the left operand is already set and not null, the assignment operation is completely bypassed. This is a critical distinction from the expression $a = $a ?? $b. The latter always performs an assignment, which can trigger unintended side effects such as invoking __set() on objects or offsetSet() on ArrayAccess implementations. The ??= operator avoids these side effects by skipping the assignment step entirely when the value is not null.
• Short-Circuit Evaluation: The operator utilizes short-circuiting. If $leftOperand is determined to be set and not null, the $rightOperand is completely ignored, preventing unnecessary execution of functions or expressions placed on the right side.
• Undefined Variable Handling: Like isset() and ??, the ??= operator safely checks the left operand. If the variable does not exist in the current scope, PHP will not emit an E_WARNING (or E_NOTICE in PHP < 8.0). It will simply initialize the variable with the right operand.
• Return Value: The expression evaluates to the final value of the left operand after the operation completes, allowing for chained assignments (e.g., $a = $b ??= $c;).

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Evaluation Matrix

$var = null;
$var ??= 'assigned'; 
// $var is now 'assigned'

unset($undefinedVar);
$undefinedVar ??= 'assigned'; 
// $undefinedVar is initialized to 'assigned'

$var = false;
$var ??= 'assigned'; 
// $var remains false (false is not null)

$var = 0;
$var ??= 100; 
// $var remains 0 (0 is not null)

$var = "";
$var ??= "default"; 
// $var remains "" (empty string is not null)