Following code review changes on #20199 (review) I reverted the mergeEnvPlaceholders. However I think I probably shouldn't have changed it as it seems to be broken now. array_merge_recursive will create this structure when the placeholder is initialized inside EnvPlaceholderParameterBag::get()

[
  'DB_DRIVER' => 
  [
    'env_DB_DRIVER_uniq_string' => 'env_DB_DRIVER_uniq_string',
  ],
]

And then once mergeEnvPlaceholders is called it results in

[
  'DB_DRIVER' =>
    [
      'env_DB_DRIVER_uniq_string' => 
      [
        0 => 'env_DB_DRIVER_uniq_string',
        1 => 'env_DB_DRIVER_uniq_string',
      ],
   ]
]

Which causes an error inside ContainerBuilder::resolveEnvPlaceholders as the placeholder is an array instead of a string.

ContextErrorException in ContainerBuilder.php line 1045: Warning: stripos(): needle is not a string or an integer

I don't think this can be fixed inside the envPlaceholders assignment because it's an issue related to the merging of two parameter bags, not the assignment of them. I would suggest using the original fix I proposed inside mergeEnvPlaceholders