Error Identifier: array.unusedOffsetFlow

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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];
	while (rand(0, 1)) {
		$a['x'] = $a['x'] + 1;
	}
	sink($a['y']);
}

Why is it reported? #

The value stored under offset 'x' of $a is read — but only by $a['x'] = $a['x'] + 1, whose result no code ever observes. Only offset 'y' reaches a sink; the value under 'x' feeds a closed computation that produces nothing.

This is different from array.unusedOffset, where the offset’s value is never read at all. Here the value flows through further computation, but that computation is itself dead, so the whole chain has no effect. This often points to a leftover accumulator or a logic error where the offset was meant to be used.

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 #

Remove the dead offset and its computation if the result is not needed:

 function doFoo(): void
 {
-	$a = ['x' => 1, 'y' => 2];
-	while (rand(0, 1)) {
-		$a['x'] = $a['x'] + 1;
-	}
+	$a = ['y' => 2];
 	sink($a['y']);
 }

Or read the offset if it was intended to be used:

 function doFoo(): void
 {
 	$a = ['x' => 1, 'y' => 2];
 	while (rand(0, 1)) {
 		$a['x'] = $a['x'] + 1;
 	}
+	sink($a['x']);
 	sink($a['y']);
 }

How to ignore this error #

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

// @phpstan-ignore array.unusedOffsetFlow
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.unusedOffsetFlow

Rules that report this error #

• PHPStan\Rules\DeadCode\UnusedVariableRule [1]