Possible Cause:
A function got declared inside of another function (i.e. “}” in the wrong place)

function listing_form_alter($form_id, &$form){
  if($form_id == 'listing_node_form'){
    //do stuff
  }

function listing_form(){
//do stuff
}//end listing_form

}//end listing_form_alter

Fix:
Move the function.

function listing_form_alter($form_id, &$form){
  if($form_id == 'listing_node_form'){
   //do stuff
  }
}//end listing_form_alter

function listing_form(){
//do stuff
}//end listing_form