Due to the way that PHP does concatenation you might occasionally come across an issue.  Create a new page called stringconcatproblem.php and paste this into the PHP block under the h1 heading:

$distance=182;    // the calculated distance of a journey in kilometres
$lengthMessage="Your journey is $distancekm";
echo $lengthMessage;

Try it.  Read the error.  You want the distance shown as 182km but PHP interprets that as part of the variable name and cannot find $distancekm.  Change the middle line to this:

$lengthMessage="Your journey is {$distance}km";

Try it.  The braces (curly brackets) separate the variable name from the other text.