Yes you can, but in you example if you want the vars $a and $b to be set to "HELLO ALL" you would write it as
$a=$b="HELLO ALL";
or if you wanted one variable name with many variables like $fruits in an array like this
$fruits= array(apple, orange, grape, pear,kiwi);
echo $fruits[0]."<br />";
echo $fruits[4]."<br />";
Which is an indexed array or you could use an associative array where you use words as key value pairs like
$vegetable = array(best=>corn,worst=>squash);
echo $vegetable['worst'];
Sorry if that is a bad example and I might have mispelled vegetable but as you can see I don't really like squash (never ate it and never will).