how to delay a specific line of code in php whilst the other line executes

Question

Im developing a ussd app which has a payment option in it.

The last message the user will receive on his phone is, "An amount of GHS 1 has been sent to your phone". This message always block the payment prompt to be received by the phone. So how do I delay the payment prompt for some seconds so that is doesn't clash with the success message ???

$transaction->updateTransactionDetails($transactionId, $reference, $request_id, SESSION_ID, MSISDN);                
echo   $RESPONSE_DATA = show("A Payment Prompt of GHS " . $totalAmount . " has been sent to your phone.", "end");
$push_payment =  $payment->momoPrompt($totalAmount,'MTN_MONEY',MSISDN,$reference, $request_id, $transactionId);

The payment prompt uses curl request