If else condition to replace value in php

I had a php script to run the if else to replace the value based on condition:

//$mobile = '23456701';
$mobile = '01234567';

$zero_lead = "/^01/";
$country_code = "44";


$phone = preg_replace($zero_lead, $country_code, $mobile);

if (strpos($phone, '/^0/') !== true) {
   
   $accountNumber = $mobile;
   
}
else
{
    $accountNumber = '';
}

$post_data = array(
                'phone' => $phone,
                'accountNumber' => $accountNumber,
            );

echo json_encode($post_data);

return: {"phone":"44234567","accountNumber":"01234567"}

my expected return if $mobile = '01234567'; will return {"phone":"44234567","accountNumber":""}

and $mobile = '23456701';will return {"phone":"23456701","accountNumber":"23456701"}

any one have suggestion?

Thanks