Hmm maybe I'm missing the point of your question but simply.
$var3 = '$var1'.'$var2';
will return the value you want i.e $var1$var2 but you are just passing in a string not doing anything dynamic. So this would work too:
$var3 = '$var1$var2';
Maybe I'm missing the point but I don't see why you are giving variables to $var1 & $var2 in order to declare another variable based on thier names. By which I mean that
Code:
<?php
$var1 = 'ONE';
$var2 = 'TWO';
$var3 = '$var1$var2';
echo $var3;
?>
will return the same as:
Code:
<?php
$var3 = '$var1$var2';
echo $var3;
?>