Some bugs here I don't know why

Let's talk Moodle in PHP

	//Get performance trend [=>Not getting desired result yet.]
	public function get_performance_trend(){
		global $DB;
		
		//Query for all enrolled course first
		$sql = "select c.id as id, c.shortname,c.category from {course} c 
					inner join {enrol} e on e.courseid = c.id
					inner join {user_enrolments} u on e.id=u.enrolid
					where u.userid=15";	 //total is 6
		$courses = $DB->get_records_sql($sql);

		$i = 0;
		$data = array();
		$sql = 'select id, name from {course_categories} where parent=0';
		$primaryLevels = $DB->get_records_sql($sql);

		foreach($primaryLevels as $category){
			$sumperformance = 0;
			$avgperformance = 0;
			$pcount = 0;
			print('category '.$category->id."\n");
			//every course avg grade
			foreach ($courses as $course){
				$flc = 0;
				//check which category the course belong (Primary 1-6)
				$flc = $this->checkFirstLevelCategory($course->category);
				//print the course we are dealing with
				print('Course with category '.$flc);
				print_r(json_encode($course));
				if($category->id == $flc){
					print("\n".'Category test passed'."\n");
					//get  user grade of this quiz				
					$sumgrade=0;
					$quizcount=0;
					$avggrade=0;
					// print('   courseid '.$course->id);

					$sql ="select id from {quiz} where course=".$course->id;
					$coursequizs = $DB->get_records_sql($sql);
					//there will be many quizs of the course.
					foreach ($coursequizs as $coursequiz){
						//select avg(sumgrades)... won't work cuz sumgrades has NULL value
						$sqlcoursequizgrade ="select sumgrades from {quiz_attempts} where quiz=".$coursequiz->id." and userid=15 order by timemodified desc limit 1";
						$grade = $DB->get_records_sql($sqlcoursequizgrade);
						// print_r($grade);
						foreach($grade as $grade){
							$sumgrade+=(float)$grade->sumgrades;
							print_r($sumgrade);
							$quizcount++;
						}//foreach grade

						if($quizcount!=0){
							$avggrade=(float)$sumgrade/$quizcount;
						}

					}//foreach quiz
					// $data[] = array('course'=>$course->shortname, 'grade'=> (float)$avggrade);
					
					$sumperformance += $avggrade;
					// print('Sum performance '.$sumperformance);
					$pcount++;
				}//end if
			}//foreach course
			if($pcount != 0){ //validate to avoid infinity error
				print('pcount '.$pcount);
				$avgperformance = $sumperformance/$pcount;
			}
			//save into the array
			$data[] = array('form'=>$category->name, 'performance'=>$avgperformance);
			$i++;
		}//foreach category

		print('Loopcount '.$i);
		if(empty($data[0])){
			$data[0] = array('form'=>"NoRecord", 'performance'=> (float)0);
		}
        
		$this->set_graphic_type('linechart');
		return $this->generate_chart(json_encode($data));//convert to json object
	}//end get_performance_trend
	//utility function for performance trend
	public function checkFirstLevelCategory($cid){		
		global $DB;
		$sql = 'select parent from {course_categories} where id='.$cid;
		$parent = $DB->get_record_sql($sql);
		if($parent->parent != 0){
			$this->checkFirstLevelCategory(($parent->parent));
		}else{
			print("\n".'cFLC() returns '.$cid);
			return $cid; 
		}
	}//end getFirstLevelCategory

why $flc is not storing the value returned

Because the method does not return the value on line 88.