Why array_unique does not detect duplicate objects?

I know that you said that you don't want converting to string, but i see that you are yet no have way out, so i propose to you use the function serialize to each object in your array, i don't found a method to compare objects that isn't converting in array or string (you cant try convert in binary or hex if you don't unfamiliar with string or array, but i don't know if you can converting to binary or hex without to convert in string).

But, if you use serialize, you can serialize the object in a read data own of php, to you compare with anothers serialized objects, this method (serialize) is safe, because you can do aunserialize, and geting the original object again.

So you can serialize all elements from array and after this, you can use array_unique, like that:

<?php

header("Content-Type: application/json");

class MyClass
{
    public $var1;
    public $var2;
    function __construct($var1, $var2)
    {
        $this->var1 = $var1;
        $this->var2 = $var2;
    }

}

$arr = [
    "a",
    "a",
    [1,2,3],
    "b",
    [1,2,3],
    new MyClass(1,1),
    new MyClass(1,new MyClass(1,1)),
    new MyClass(1,new MyClass(1,1)),
];

$arrSerilized = array_map("serialize", $arr);

var_dump(
    array_map(
        "unserialize",
        array_unique(
            $arrSerilized,
            SORT_STRING
        )
    )
);

/* output:
array(5) {
    [0]=>
    string(1) "a"
    [2]=>
    array(3) {
        [0]=>
        int(1)
        [1]=>
        int(2)
        [2]=>
        int(3)
    }
    [3]=>
    string(1) "b"
    [5]=>
    object(MyClass)#6 (2) {
        ["var1"]=>
        int(1)
        ["var2"]=>
        int(1)
    }
    [6]=>
    object(MyClass)#7 (2) {
        ["var1"]=>
        int(1)
        ["var2"]=>
        object(MyClass)#8 (2) {
            ["var1"]=>
            int(1)
            ["var2"]=>
            int(1)
        }
    }
}
*/

Hope this help you man, have a good day!

P.S.: With serialize you can preserve same value in different variable type, like 1 and "1" are serialized in different read data of php

Without knowing about your entity class its hard to guess why this is happening. But I guess your main issue here is __toString() method . If you have not defined it, you should add one such that it returns a unique/distinct string for each entity object. If its already defined make sure it returns distinct string.

class User{ 
   private $name;

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

   function __toString(){ 
     return $this->name; 
   }
}

$user = [];
$users[] = new User("User1");
$users[] = new User("User2");
$users[] = new User("User3");

$user1= $users[0];
$users[]=$user1; //duplicate

echo(count(array_unique($users))); // output should be 3

Given the limited information about entity class I can guess this far.

Edit:

After reading your edits I guess you are locking yourself into this. Since array_unique will try to convert an entity object to either string or number depending on the sort_flag you pass. More on array_unique. So either you need to implement __toString() or add some public properties which define the uniqueness of your object to entity e.g

class User{ 
       public $id;
       private $name;

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

$user = [];
$users[] = new User(1,"User1");
$users[] = new User(2,"User2");
$users[] = new User(3,"User3");

$user1= $users[0];
$users[]=$user1; //duplicate
echo(count(array_unique($users, SORT_REGULAR))); // output should be 3

Please note the public property $id and SORT_REGULAR flag.