Prevent non-JSON stdout output from corrupting MCP tool responses

[OmarFaruk-0x01]

Problem

When the MCP tool subprocess produces any non-JSON output on stdout before the actual JSON response, the MCP client fails to parse the response:

My Case

Running Laravel Boost MCP on PHP 8.4 with spatie/laravel-comments v1.7.3 installed. PHP 8.4 deprecates implicitly nullable parameters, and this package uses that pattern. Calling any Boost MCP tool (e.g., application-info) produced:

Deprecated: Spatie\Comments\Models\Concerns\HasComments::subscribers(): Implicitly marking parameter $type as nullable is deprecated...

Deprecated: Spatie\Comments\Models\Concerns\HasComments::comment(): Implicitly marking parameter $commentator as nullable is deprecated...

{"isError":false,"content":[...]}

The deprecation text prefixed the JSON, breaking the MCP protocol entirely.

Changes

1. Added ini_set('display_errors', 'stderr') to redirect PHP errors to stderr instead of stdout during tool execution.

2. Added extractJson() safety net that strips any leading non-JSON content from stdout before parsing.