error_reporting(E_ALL|E_STRICT);echo $var;  //display an unset variable. A notice will appear.
 
//using an unset variable inside an if()-condition will also display the notice message
//i use this kind of checking all the time, but i think it would be better to use
//isset() since E_ALL|E_STRICT makes your code "cleaner"
 
if ($var) {
    echo $var;
}
 
//we can avoid this if we use isset()
if(isset($var)) {
    echo $var;
}
 
//so before using a variable, i would set it to '' on the top part of my code
$var = '';
echo $var;
//no notice message will be displayed.

$str1 = ''; //Street Address
$str2 = ''; //Building Number
$str3 = ''; //Country
...the rest of my code...