从数组中获取帐号

I have a variable $accountNumber which is getting an array like this:

I want to get the accountNumber string from this array and put it inside another variable called $ac.

Basically what I am trying to achieve is that $ac will have the string fi4240553120051164 so I can just pass it on to my database.

Basically the code is like

$accountNumber = $bankReport->getAccountNumbersVerified();
// Now get the accountNumber string from $accountNumber and put it into $ac
$ac = //code here 

Very Very simple.

$ac = $accountNumber[0]->accountNumber;