Narrow string type through variable assigned from strlen()/mb_strlen() via conditional expression holders

[phpstan-bot]

Summary

When strlen() or mb_strlen() result is assigned to a variable and that variable is later compared (e.g. $len > 0), PHPStan now propagates the narrowing back to the original string argument. Previously, only direct comparisons like strlen($str) > 0 narrowed the string type; an intermediate variable broke the narrowing chain, causing false positives like "Offset 0 might not exist on ''|':'".

Changes

• src/Analyser/TypeSpecifier.php: Added conditional expression holders for $var = strlen($str) and $var = mb_strlen($str) assignments in the null (assignment) context. Two holders are created per assignment:

  • If $var >= 1 (int<1, max>), narrow $str to non-empty-string
  • If $var >= 2 (int<2, max>), narrow $str to non-falsy-string

  Added new private method createStrlenConditionalExpressionHolders() following the same pattern as createArrayDimFetchConditionalExpressionHolder().

Root cause

When code like this was analysed:

$inputLen = strlen($input);
$hasTrailingColon = $inputLen > 0 && $input[$inputLen-1] === ':';

The TypeSpecifier already had handlers for narrowing strings when strlen() appears directly in a comparison (e.g. strlen($str) > 0). However, when the result was assigned to an intermediate variable first, the connection between the variable and the strlen() call was lost. The comparison $inputLen > 0 only narrowed $inputLen (to int<1, max>), not $input.

The fix uses PHPStan's existing ConditionalExpressionHolder mechanism to create a deferred relationship at assignment time: "if $inputLen is later narrowed to int<1, max>, then also narrow $input to non-empty-string". This mechanism is already used successfully for array_key_first(), array_search(), and similar functions.

Analogous cases probed

• mb_strlen(): Included in the fix with the same narrowing rules
• count()/sizeof(): Already works through variables (verified — has existing narrowing mechanism)
• Re-assignment: Verified that re-assigning the variable ($len = 5) correctly invalidates the conditional holders
• Various comparison operators: > 0, >= 1, !== 0, >= 2, > 1 all work correctly through the holder mechanism
• Non-variable string args: Property access ($obj->name) as the strlen argument also narrows correctly

Test

• NSRT test (tests/PHPStan/Analyser/nsrt/bug-13688.php): Tests type narrowing through variable for constant string unions, non-empty-string, non-falsy-string, mb_strlen, !== 0 comparison, === 0 non-narrowing, and re-assignment invalidation.
• Rule test (tests/PHPStan/Rules/Arrays/data/bug-13688.php + NonexistentOffsetInArrayDimFetchRuleTest::testBug13688): Verifies the original false positive "Offset 0 might not exist" is no longer reported.

Fixes phpstan/phpstan#13688