> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# PHP Null Coalescing Assignment
The `??=` (null coalescing assignment) operator evaluates its left operand and assigns the value of its right operand to it strictly if the left operand is `null` or undefined. If the left operand holds any non-null value, the assignment is bypassed entirely, and the right operand is not evaluated.
Introduced in PHP 7.4, it provides a syntactic shorthand for conditional initialization based on nullability.
## Syntax
```php theme={"dark"}
$leftOperand ??= $rightOperand;
```
## Logical Equivalence
The internal behavior of the null coalescing assignment operator is functionally equivalent to a conditional `isset()` check.
```php theme={"dark"}
// Using the null coalescing assignment operator
$a ??= $b;
// Is functionally equivalent to the following control structure:
if (!isset($a)) {
$a = $b;
}
```
## Technical Characteristics
* **Single Evaluation:** The operator evaluates its left operand exactly once. If the left operand is a complex expression (such as a function returning a reference, or a property access), it is not evaluated twice (once to read, once to write).
* **Assignment Bypass:** If the left operand is already set and not `null`, the assignment operation is completely bypassed. This is a critical distinction from the expression `$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;`).
## Evaluation Matrix
The operator strictly checks for `null`. It does not perform truthiness checks. Falsy values such as `0`, `false`, `""` (empty string), or `[]` (empty array) will prevent the assignment.
```php theme={"dark"}
$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)
```
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