php - Using citation marks around a variable in array access -

i'm reading som legacy code , come on curious case:

$my_assoc_array; /* user defined associative array */ $my_key; /* user defined string */ $value = $my_assoc_array["$my_key"]; 

is there clever reason why want have citation marks (") around variable when it's used key? special corner case? or there no reason @ this?

-- edit --
maybe in old version of php there difference? (remember legacy code).

there 1 example can find output differs when $mykey = false. (which perhaps not apply example $mykey string, again: wild wild world of php)

<?php $arr = array("1"=>"b", "0"=>"a"); $mykey = false; var_dump($arr[$mykey]); // returns "a" var_dump($arr["$mykey"]); // gives undefined index error  $mykey = true; var_dump($arr[$mykey]); // returns "b" var_dump($arr["$mykey"]); // returns "b" 

what can (mis-)used beats me...