Curl is not printing the random number generated by rand() function

I am trying to send a OTP using Ringcaptcha and using the following code.

$randnum = rand(100000,1000000);
$cmd = 'curl -k -X POST --data-urlencode "api_key=xxxxxxxxx" --data-urlencode "phone=+911234567890" --data-urlencode code=$randnum http://ift.tt/2zr7DoJ';
exec($cmd,$result);

this code doesn't out put a random number everytime.

$echo randnum 

outputs a different number everytime, but curl doesn't do that.

I tried all the combinations like ' , " and "code=".$randnum etc. Can somebody help me?