Error Identifier: array.offsetOverwritten

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Every error reported by PHPStan has an error identifier. Here’s a list of all error identifiers. In PHPStan Pro you can see the error identifier next to each error and filter errors by their identifiers.

Code example #

<?php declare(strict_types = 1);

/** @param mixed $v */
function sink($v): void
{
}

function doFoo(): void
{
	$a = ['x' => 1, 'y' => 2];
	$a['x'] = 3;
	sink($a);
}

Why is it reported? #

The array assigned to $a stores the value 1 under offset 'x', but $a['x'] = 3 replaces it before any code reads it. The original value under 'x' is a dead store: it is computed and kept, but no code observes it.

Unlike variable.unused, the array $a itself is used; only this particular offset of the assigned array is redundant. Unlike array.unusedOffset, the offset’s value is not just left unread — a later write overwrites it first. This often points to a leftover key or a logic error, such as writing to the wrong offset.

This rule is part of PHPStan’s dead code analysis. It is reported at rule level 4 and above, and is currently part of Bleeding Edge.

How to fix it #

Put the final value directly into the array literal:

 function doFoo(): void
 {
-	$a = ['x' => 1, 'y' => 2];
-	$a['x'] = 3;
+	$a = ['x' => 3, 'y' => 2];
 	sink($a);
 }

Or remove the offset from the literal if it is always set later:

 function doFoo(): void
 {
-	$a = ['x' => 1, 'y' => 2];
+	$a = ['y' => 2];
 	$a['x'] = 3;
 	sink($a);
 }

If the later write targets the wrong offset, fix the offset name:

 	$a = ['x' => 1, 'y' => 2];
-	$a['x'] = 3;
+	$a['z'] = 3;
 	sink($a);

How to ignore this error #

You can use the identifier array.offsetOverwritten to ignore this error using a comment:

// @phpstan-ignore array.offsetOverwritten
codeThatProducesTheError();

You can also use only the identifier key to ignore all errors of the same type in your configuration file in the ignoreErrors parameter:

parameters:
	ignoreErrors:
		-
			identifier: array.offsetOverwritten

Rules that report this error #

• PHPStan\Rules\DeadCode\UnusedVariableRule [1]