打印成员之间的关系

Question

I've a university project in which I've to print the relations between students in different classes level by level. The idea is if we have John and Kris studying in the same class they are friends of first level, if Kris studies with Math in same class then John and Math are friends of second level. I researched the problem and I found algorithms like this, but my main problem is that I use objects as input data :

<?php
class Student {

 private $id = null;
 private $classes = [];

 public function __construct($id) {
   $this->id = $id;
 }

 public function getId() {
   return $this->id;
 }

 public function getClasses() {
   return $this->classes;
 }

 public function addClass(UClass $class) {
   array_push($this->classes, $class);
 }

}

class UClass {

 private $id = null;
 private $students= [];

 public function __construct($id) {
   $this->id = $id;
 }

 public function getId() {
   return $this->id;
 }

 public function getStudents() {
   return $this->students;
 }

 public function addStudent(Student $student) {
   array_push($this->students, $student);
   $student->addClass($this);
 }

}

function getRelations(Student $start_student, &$tree = array(), $level = 2, &$visited) {
 foreach ($start_student>Classes() as $class) {    
   foreach ($class->Students() as $student) {
     if($start_student->getId() != $student->getId() && !is_int(array_search($student->getId(), $visited))) {
         $tree[$level][] = $student->getId();
         array_push($visited, $student->getId());
         getRelations($student, $tree, $level+1, $visited); 
     }
   }
 }
}

$class = new UClass(1);
$class2 = new UClass(2);
$class3 = new UClass(3);

$student = new Student(1);
$student2 = new Student(2);
$student3 = new Student(3);
$student4 = new Student(4);
$student5 = new Student(5);
$student6 = new Student(6);

$class->addStudent($student);
$class->addStudent($student2);
$class->addStudent($student4);

$class2->addStudentr($student2);
$class2->addStudent($student4);
$class2->addStudent($student5);

$class3->addStudent($student4);
$class3->addStudent($student5);
$class3->addStudent($student6);

$tree[1][] = $student->getId();
$visited = array($student->getId());
getRelations($student, $tree, 2, $visited);
print_r($tree);

I'm stuck at writing getRelations() function that should create an array that is something like

Array ( [1] => Array ( [0] => 1 ) [2] => Array ( [0] => 2 [1] => 4 ) [3] => Array ( [0] => 5 [1] => 6 ) ) 

but I can't get the recursion right(or probably the whole algorithm). Any help will be greatly appreciated.

Answer 1

I come up with that function(not sure if it's the best solution, but it works with the class objects)

function print_students(Student $start_student, &$tree = array(), $lvl = 1) {
  if (!$start_student) {
    return;
  }
  $tree[$lvl][] = $start_student->getId();
  $q = array();
  array_push($q, $start_student);
  $visited = array($start_student->getId());

  while (count($q)) {
    $lvl++;
    $lvl_students = array();
    foreach ($q as $current_student) {
      foreach ($current_student->getClasses() as $class) {
        foreach ($class->getStudents() as $student) {
          if (!is_int(array_search($student->getId(), $visited))) {
            array_push($lvl_students, $student);
            array_push($visited, $student->getId());
            $tree[$lvl][] = $student->getId();
          }
        }
      }
    }
    $q = $lvl_students;
  }
}