Summary

When iterating over a constant string array like ['need', 'field'] in a foreach loop, type narrowing from isset() and is_string() checks inside the loop body was not preserved after the loop. This meant PHPStan couldn't verify that the array had the expected shape after the loop completed.

Changes

• Added constant array foreach unrolling in src/Analyser/NodeScopeResolver.php (after the existing foreach analysis): when the iterable is a small constant array that iterates at least once, the loop body is re-processed once per element with specific constant key/value types, accumulating type narrowing
• Added applyNarrowingsFromForeachUnroll() method to src/Analyser/MutatingScope.php: selectively transfers only HasOffsetType and HasOffsetValueType accessory types from the unrolled scope to the final scope, without affecting base array structure (avoiding interference with array mutations in the loop body)
• Added regression test in tests/PHPStan/Analyser/nsrt/bug-11533.php

Root cause

PHPStan's loop analysis uses fixed-point iteration where the loop variable holds the union of all possible values (e.g., 'need'|'field'). When isset($param[$field]) is evaluated with this union-typed key, the TypeSpecifier can only produce a NonEmptyArrayType narrowing (not per-key HasOffsetType), because narrowing for all union members would be unsound in non-loop contexts. Similarly, is_string($param[$field]) narrows the expression $param[$field] but can't decompose this into per-key HasOffsetValueType constraints on $param.

The fix adds a post-loop unrolling pass specifically for constant arrays: the loop body is re-analyzed once per element with the specific constant value, so isset($param['need']) and is_string($param['need']) produce the precise per-key narrowing. Only accessory types (HasOffsetType/HasOffsetValueType) are transferred from the unrolled scope to the final scope, ensuring array mutations in the loop body are unaffected.

Test

The regression test verifies that after foreach (['need', 'field'] as $field) { if (!isset($param[$field]) || !is_string($param[$field])) throw; }, the type of $param is correctly narrowed to non-empty-array<mixed>&hasOffsetValue('field', string)&hasOffsetValue('need', string). Tests with and without a key variable are included, as well as a function call test matching the original issue.

Fixes phpstan/phpstan#11533