fix: keep valueless query parameters when casting to a string

Valueless query parameters (for example ?foo) are stored as null, and has()
reports them as present, but __toString() passed those null values to
http_build_query() which skips them entirely, dropping the parameters from the
output. Render null values as empty-valued parameters so they survive a
round-trip.

PR.md

@@ -0,0 +1,72 @@
+# fix: keep valueless query parameters when casting to a string
+
+## Problem
+
+A query parameter that has no value (for example `?foo` or `?foo=`) is parsed and
+stored, and `has()` / `getAllQueryParameters()` report it as present, but casting
+the bag (or the `Url`) back to a string silently drops it:
+
+```php
+$url = Spatie\Url\Url::fromString('https://example.com/?foo=');
+$url->hasQueryParameter('foo');   // true
+(string) $url;                    // "https://example.com"  <-- "foo" is gone
+```
+
+So `hasQueryParameter('foo')` is `true` while the parameter disappears from the
+string representation, and the URL does not round-trip.
+
+## Root cause
+
+`src/QueryParameterBag.php`. `fromString()` deliberately stores valueless
+parameters as `null` (added in #11, and pinned by the "empty query parameters
+default to null" test):
+
+```php
+$parameters = array_map(fn ($param) => $param !== '' ? $param : null, $parameters);
+```
+
+but `__toString()` passes those `null` values straight to `http_build_query()`:
+
+```php
+return http_build_query($this->parameters, '', '&', PHP_QUERY_RFC3986);
+```
+
+`http_build_query()` skips `null` values entirely, so every valueless parameter
+is removed from the output.
+
+## The fix
+
+Render `null` values as empty-valued parameters before building the query string,
+so they survive the round-trip:
+
+```php
+$parameters = array_map(fn ($value) => $value ?? '', $this->parameters);
+
+return http_build_query($parameters, '', '&', PHP_QUERY_RFC3986);
+```
+
+`?foo` / `?foo=` now serialize back to `foo=` (which parses to the same value),
+and parameters with an actual value or an array value are unaffected.
+
+## Tests
+
+Added two cases to `QueryParameterBagTest`:
+
+- `it keeps valueless parameters when being casted to a string`
+  (`fromString('offset')` -> `has('offset')` true and `__toString()` === `offset=`).
+- `it keeps valueless parameters alongside valued ones when being casted to a string`
+  (`fromString('foo=&bar=baz')->__toString()` === `foo=&bar=baz`).
+
+## Gate
+
+```
+composer install && vendor/bin/pest
+```
+
+(`phpunit` is intentionally blocked by this package in favour of Pest.)
+
+Result: exit code `0` - `Tests: 110 passed`.
+
+Counterfactual: reverting `__toString()` to pass `$this->parameters` directly to
+`http_build_query()` makes both new tests fail (the valueless parameters are
+dropped from the output).

src/QueryParameterBag.php

@@ -65,6 +65,12 @@ class QueryParameterBag implements \Stringable
 
     public function __toString(): string
     {
-        return http_build_query($this->parameters, '', '&', PHP_QUERY_RFC3986);
+        // Parameters without a value are stored as null (for example "?foo"),
+        // but http_build_query() skips null values entirely, which would drop
+        // parameters that has() still reports as present. Render them with an
+        // empty value so they survive a round-trip.
+        $parameters = array_map(fn ($value) => $value ?? '', $this->parameters);
+
+        return http_build_query($parameters, '', '&', PHP_QUERY_RFC3986);
     }
 }

tests/QueryParameterBagTest.php

@@ -89,6 +89,21 @@ it('can be casted to a string', function () {
 });
 
 
+it('keeps valueless parameters when being casted to a string', function () {
+    $queryParameterBag = QueryParameterBag::fromString('offset');
+
+    expect($queryParameterBag)->has('offset')->toBeTrue();
+    expect($queryParameterBag)->__toString()->toEqual('offset=');
+});
+
+
+it('keeps valueless parameters alongside valued ones when being casted to a string', function () {
+    $queryParameterBag = QueryParameterBag::fromString('foo=&bar=baz');
+
+    expect($queryParameterBag)->__toString()->toEqual('foo=&bar=baz');
+});
+
+
 it('can be created from a string with url encoded values', function () {
     $queryParameterBag = QueryParameterBag::fromString(
         'category=storage%20furniture&discount=%3E40%25%20off&range%5B0%5D=10&range%5B1%5D=20'