Might've found the answer here:
http://2lemetry.com/2013/04/17/a-simple-example-arduino-mqtt-m2mio/
// publish light reading every 5 seconds
if (millis() > (time + 5000))
{
time = millis();
String pubString = "{"report":{"light": "" + String(lightRead) + ""}}";
pubString.toCharArray(message_buff, pubString.length()+1);
//Serial.println(pubString);
client.publish("io.m2m/arduino/lightsensor", message_buff);
}
In this example, rightRead is an int. The other parts of the message_buff are part of how this particular person is using the data, but the important part is this:
String(lightRead)
So this String() function converts whatever is sent to it as a parameters into a string.