Script php artisan optimize handling the post-update-cmd event returned with error code 255

I am getting an error when I run composer update:

Generating autoload files
> Illuminate\Foundation\ComposerScripts::postUpdate
> php artisan optimize

                                                           
  [Symfony\Component\Debug\Exception\FatalErrorException]  
  Call to undefined function apache_request_headers()      
                                                           

Script php artisan optimize handling the post-update-cmd event returned with error code 255

Problem is in my custom error handler, loaded through a service provider:

private function include_request_headers()
{
    // Include request headers in log entry
    if( $request_headers = \apache_request_headers() )
    {
        $this->log_entry .= 'Request headers:' . PHP_EOL;

        foreach( $request_headers as $k => $v )
        {
            $this->log_entry .= "\t" . $k . ' = ' . $v . PHP_EOL;
        }
    }
}

Service Provider:

<?php

namespace App\Providers;

use Illuminate\Support\ServiceProvider;
use App\Exceptions\CustomErrorHandler;
use App;

class CustomErrorHandlingProvider extends ServiceProvider
{
    /**
     * Bootstrap the application services.
     *
     * This section waits until all of the service providers 
     * have been registered in the register method.
     *
     * @return void
     */
    public function boot()
    {
        //
    }

    /**
     * Register the application services.
     *
     * @return void
     */
    public function register()
    {
        if( App::environment('local', 'production') )
        {
            error_reporting(0);
            new CustomErrorHandler;
        }
    }
}

I realize that this may not be proper usage of a service provider, since I'm not actually binding something to the container.

The apache_request_headers function definitely exists on my server. I had to add the backslash in front of it, because without it the error above thinks apache_request_headers is part of the App\Exceptions namespace.

I did move the loading of this service provider to the top of the stack in config/app.php, because I want to have it load as soon as possible. If I comment out the section that is throwing the error, composer update works fine, and even with uncommenting the error handler works fine too.

Any idea why this error is happening?