Looks good to me.
An env var is always a string anyway, so supporting JSON here makes sense.

You could add JSON_THROW_ON_ERROR and fail clearly on invalid JSON,

Very close to #45535.

Did you consider populating $_SERVER['APP_RUNTIME_OPTIONS'] in your index.php file instead? Just wondering about alternatives:

--- a/public/index.php
+++ b/public/index.php
@@ -4,6 +4,10 @@ use App\Kernel;
 
 require_once dirname(__DIR__).'/vendor/autoload_runtime.php';
 
+if (is_string($_SERVER['APP_RUNTIME_OPTIONS'] ??= $_ENV['APP_RUNTIME_OPTIONS'] ?? [])) {
+    $_SERVER['APP_RUNTIME_OPTIONS'] = json_decode($_SERVER['APP_RUNTIME_OPTIONS'], true, JSON_THROW_ON_ERROR);
+}
+
 return function (array $context) {
     return new Kernel($context['APP_ENV'], (bool) $context['APP_DEBUG']);
 };

Did you consider populating $_SERVER['APP_RUNTIME_OPTIONS'] in your index.php file instead? Just wondering about alternatives:

--- a/public/index.php
+++ b/public/index.php
@@ -4,6 +4,10 @@ use App\Kernel;
 
 require_once dirname(__DIR__).'/vendor/autoload_runtime.php';
 
+if (is_string($_SERVER['APP_RUNTIME_OPTIONS'] ??= $_ENV['APP_RUNTIME_OPTIONS'] ?? [])) {
+    $_SERVER['APP_RUNTIME_OPTIONS'] = json_decode($_SERVER['APP_RUNTIME_OPTIONS'], true, JSON_THROW_ON_ERROR);
+}
+
 return function (array $context) {
     return new Kernel($context['APP_ENV'], (bool) $context['APP_DEBUG']);
 };

"Funny", I currently only used the bin/console so I added a similar check in my bin/console file but indeed, I will certainly have the same need in the index.php.

I think adding this directly in Symfony runtime will

• avoid people to add this themself
• avoid having to add this on different php files (index.php, bin/console, another ?)

I'm only sad about the fact it's obviously considered as a new feature and I'll have to wait 7.4 ^^

(please add a line in the changelog of the component)

(please add a line in the changelog of the component)

Updated

Thank you @VincentLanglet.