Problem with callback function in ESP32 wifi library

Hi,

I'm using ESP3, but it is not accepting callback functions. can anyone suggest me to resolve the issue so that i can able to gwt command from IBM cloud to ESP32.

sketch_oct20a:42:35: error: 'callback' was not declared in this scope
PubSubClient client(server, 1883, callback, wifiClient);
^
Multiple libraries were found for "WiFi.h"
Used: C:\Users\I2785\AppData\Local\Arduino15\packages\esp32\hardware\esp32\1.0.6\libraries\WiFi
Not used: C:\Program Files (x86)\Arduino\libraries\WiFi
exit status 1
'callback' was not declared in this scope

the compiler gave you a clue...

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