phpstan-type-trace

Trace any variable's type history through your code — no more guessing why PHPStan thinks it's mixed.

Above: the type evolution of $modelType in real larastan code. Five events, one command, zero source edits.

Install

composer require --dev kayw-geek/phpstan-type-trace

Auto-registered via phpstan-extension-installer. Otherwise add to phpstan.neon:

includes:
    - vendor/kayw-geek/phpstan-type-trace/extension.neon

Two ways to use it

1. CLI — inspect any line, no source edits

./vendor/bin/phpstan-trace inspect src/Foo.php:42 myVar

Output:

$myVar · doStuff [src/Foo.php] (up to L42)

  L18  param    int|null
  L25  assign   int
  L31  read     positive-int
  L42  read     positive-int

4 events · final type: positive-int

Variable name is optional — if only one variable has events at the target line, it's auto-picked. Otherwise the candidates are listed.

Pass --json for machine-readable output (handy for tooling).

2. traceType() — drop in a marker, get the chain on your next phpstan run

No extra command. Just call traceType($var) anywhere, then run vendor/bin/phpstan analyse like you always do — the chain shows up as a phpstan error at that line.

function compute(?float $discount = null): float
{
    $discount ??= 0.1;
    traceType($discount, 'after ??=');
    return 1 - $discount;
}

 ------ -----------------------------------------------------------
  Line   PriceCalculator.php
 ------ -----------------------------------------------------------
  5      Type chain for $discount in compute — after ??=
           L3   param      float|null
           L4   assign-op  float
 ------ -----------------------------------------------------------

traceType() is a runtime no-op (autoloaded from src/runtime.php), so leaving a stray call in production code does nothing — it only emits during static analysis.

Use it with Claude Code

When Claude Code (or any LLM agent) is chasing PHPStan errors, it usually guesses at types. With this extension installed as a Claude Code plugin, Claude invokes the trace automatically — fixes are grounded in real upstream type evidence, not pattern-matching.

/plugin marketplace add kayw-geek/phpstan-type-trace
/plugin install phpstan-type-trace@kayw-geek

Installed into ~/.claude/plugins/cache/, auto-discovered across every project. Updates: /plugin marketplace update kayw-geek then reinstall.

Signature

function traceType(mixed $value, ?string $reason = null): void

• $value — a variable, property fetch ($this->x), or static property (Foo::$bar). For arbitrary expressions, only the snapshot type is printed.
• $reason — optional label shown in the chain header. String literal only.

What gets captured

Source	Origin label	Example
Function/method params	param	function f(int $x)
Closure / arrow-fn params	param	fn(int $x) => ...
Variable assignment	assign	$x = 5;
Compound assignment	assign-op	$x += 1; $x ??= 'def';
Reference assignment	assign-ref	$x = &$other;
Property fetch	read	$this->foo
Static property fetch	read	Foo::$bar
Variable read	read	bare $x usage

Narrowing via if, instanceof, === null, is_*, early-returns is free — PHPStan's Scope is already narrowed by the time collectors run.

Limitations

• Loops report the post-fixpoint type, not per-iteration deltas.
• Multiple closures inside the same enclosing function share one bucket. Same-named vars across sibling closures may collide.
• Cannot follow values across function boundaries.
• Ref-aliases ($alias = &$x; $alias[] = 'y';) show only the snapshot at the call.

How it works

Two-phase PHPStan pipeline:

1. Collectors (one per event kind) record every relevant AST event with (file, functionKey, path, line, type, origin):
   • Param entry: ParamInFunctionCollector, ParamInMethodCollector, ParamInClosureCollector, ParamInArrowFunctionCollector — hooked on PHPStan's In*Node virtual nodes so scope is already inside the function when params are read.
   • Reads: VarReadCollector, PropertyFetchCollector, StaticPropertyFetchCollector.
   • Writes: AssignCollector, AssignOpCollector (covers all 13 compound-op subclasses), AssignRefCollector.
   • Call sites: TraceCallCollector.
2. TraceReportRule runs once at the end on the virtual CollectedDataNode. For each traceType() call it joins the recorded events on (functionKey, path) filtered to lines <= the call line, sorts by line (mutations win on ties), collapses only boring repeated reads of the same type, and emits the delta chain as a PHPStan error.

The CLI runs the same pipeline with a dump env var set, captures every chain as a JSON sentinel error, then filters to the (file, line, variable) you asked about.

License

MIT