How would you test PHP code which uses global variable?

Suppose the class Account (in account.php file) which uses a $DB variable to store data in the database. The $DB variable is initialized in another globals.php file.

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/**** globals.php ****/

$DB = PDO (....);

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/**** account.php ****/

public function create($data) { global $DB; .... }

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Now, suppose that you want to test a function in account class called create using PHPUnit test. How would you initialize the $DB object?