PHP Define index for HTML form $_POST -

i have following php code

<?php     if(isset($_post['address_building_number']) or !isset($_post['address_building_number'])){         $address_building_number = $_post['address_building_number'];         echo 'hello';         $good = 'good bye';     }     echo $address_building_number;     echo $good;  ?> 

when hit submit button words hello , bye displayed telling me php notice: undefined index: $_post['address_building_number' html input form name address_building_number.

how can modify if(isset($_post['address_building_number']) or !isset($_post['address_building_number'])){ can 'define index' while still able echo hello , bye.

edit

all-in-one version

<?php     if(isset($_post['address_building_number']) || !empty($_post['address_building_number'])){         $address_building_number = $_post['address_building_number'];         echo 'hello';         $good = 'good bye';     }     echo $address_building_number;     echo $good;      // comment out exit; if want form not appear after submit     // exit; ?>  <form action="" method="post">    name: <input type="text" name="address_building_number"><br>    <input type="submit" value="submit"> </form> 

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original answer

your conditionals seem fighting against themselves.

notice if(isset , if(!isset $_post['address_building_number']

what happening here is, "if is set , if not set"; being contradictory statement.

what meant use was:

if(isset($_post['address_building_number']) or !empty($_post['address_building_number']))  

(quick fyi) if that's case, || has precedence on or therefore use:

if(isset($_post['address_building_number']) || !empty($_post['address_building_number'])) 

...which checks see if set, , if not empty.

the following has been tested , echo'ed no error messages:

html

<form action="action.php" method="post">    name: <input type="text" name="address_building_number"><br>    <input type="submit" value="submit"> </form> 

php (action.php) example

<?php     if(isset($_post['address_building_number']) || !empty($_post['address_building_number'])){         $address_building_number = $_post['address_building_number'];         echo 'hello';         $good = 'good bye';     }     echo $address_building_number;     echo $good; ?> 

in regards "so can 'define index' while still able echo hello , bye" - i'm having hard time understanding mean this.

if mean is, "if can still echo "hello , bye" outside of conditional statement", answer yes; answer does, since it's using or (||) operator. had been and (&&) operator, not possible.