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drupal/core 11.1

DrupalGetHeadersAssocArrayRector

Convert drupalGet() $headers from indexed colon-separated strings or null values to an associative array, as required by Drupal 11.1.0.

-$this->drupalGet('/path', [], ['X-Requested-With: XMLHttpRequest']);
-$this->drupalGet('', [], ['Accept-Language' => NULL]);
+$this->drupalGet('/path', [], ['X-Requested-With' => 'XMLHttpRequest']);
+$this->drupalGet('', [], ['Accept-Language' => '']);

Configure your rector.php:

<?php

use DrupalRector\Drupal11\Rector\Deprecation\DrupalGetHeadersAssocArrayRector;
use Rector\Config\RectorConfig;

return RectorConfig::configure()
    ->withRules([
        DrupalGetHeadersAssocArrayRector::class,
    ]);

Other rules in the drupal/core 11.1 set