replace value in array if a field is empty

I am new with php so this might be a silly question. I have this code..

$message = str_replace( array( '%TITLE%', '%VOUCHER%', '%BUYERUSER%', '%DEALPRICE%', '%DIFFERENCE%', '%DATEOFPURCH%' ), array( $offer->post_title, $voucher_code, $user->user_nicename, $offer->deal_owner_price, $offer->deal_sale_price - $offer->deal_owner_price, date("d/m/y") ), $message ); 

Code (markup):

now I would like to set the value of '%DIFFERENCE%' to a string saying not available or something, if $offer->deal_sale_price field is left empty. Any help please?

Thanks.

 

Easiest way would be to check the value before using it. Something like this (before the str_replace, just swap the $offer->deal_sale_price with $offer_price in the array) :
$offer_price = (empty($offer->deal_sale_price) ? 'not available' : $offer->deal_sale_price);

 

Really sorry but I didnt quite get how the end code should be. I am completely new in php so please bear with me

 

$offer_price = (empty($offer->deal_sale_price) ? 'not available' : $offer->deal_sale_price - $offer->deal_owner_price);

$message = str_replace( array( '%TITLE%', '%VOUCHER%', '%BUYERUSER%', '%DEALPRICE%', '%DIFFERENCE%', '%DATEOFPURCH%' ), array( $offer->post_title, $voucher_code, $user->user_nicename, $offer->deal_owner_price, $offer_price, date("d/m/y") ), $message ); 

PHP:

That should do it.

 

worked beautifully mate. Thanks a lot