javascript - Issue calling JS function from PHP echo with a specific variable

I have a JS function that will add keys with values to the current URL. There are two almost identical links below, only difference is the variable being passed to the JS function. One link passes $month, the other passes $event_category. For some reason, when passing $event_category, the JS function doesn't even get called. Anyone know what I'm doing wrong?

You'll have to scroll to the right to see where the difference is.

  $month = 1;
  $event_category = (string) ($eventCategories[$k]["event_category"]);

  echo gettype($event_category); // prints "string"


  // doesn't work?
  echo '<div ><a href="javascript:void(0);" onclick="javascript:insertParam('. "'event_category'" .', '. $event_category .');" role="button">
  <image width="100" height="60" src="images/'. $images_list[$eventCategories[$k]["event_category"]].'"></a></div>'; 

  // works
  echo '<div ><a href="javascript:void(0);" onclick="javascript:insertParam('. "'event_category'" .', '. $month .');" role="button">
  <image width="100" height="60" src="images/'. $images_list[$eventCategories[$k]["event_category"]].'"></a></div>';   
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