Manual:$wgWhitelistAccount

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User rights, access control and monitoring: $wgWhitelistAccount
Types of accounts that can create new users.
Introduced in version:1.1.0
Removed in version:1.5.0
Allowed values:(array of keys and boolean values)
Default value:array ( 'user' => 1, 'sysop' => 1, 'developer' => 1 )

Details

Types of accounts that can create new users. (1 for true, 0 for false)

It's not 100% safe, there could be security hole using that one. Use at your own risks.

This setting was deprecated in version 1.5.0, and replaced by $wgGroupPermissions .