stop update parent if he has child - php

Question

I have list like this
John
Jack
Husam
Koko
Rami
Loay
And I have this function that allow me to change father of child, in my case I can change Koko to make him child of Loay.
I want to stop this because he has child, how can I validate this case to check if he has child then can't change it, and if he does not has child then I can change his father ?
This is my function
public function updateParentId($parentId, $childId)
{
$statment = $this->db->prepare("UPDATE person SET parent = $parentId WHERE id = $childId");
$statment->execute();
$result = $statment->rowCount();
if($result == "1")
{
$message = '<label>successfully</label>';
}
else
{
$message = '<label>Wrong</label>';
}
echo $message;
}
}

Answer 1

Following query will return all children below Koko(Rami, Loay) in hierarchy.
$statment = $this->db->prepare('SELECT id, parent FROM (SELECT * FROM person ORDER BY parent, id) sorted, (SELECT #id:=:id) temp WHERE (FIND_IN_SET(parent, #id) > 0 || id = "Loay") AND #id:=CONCAT(#id, ",", id)');
$statment->execute();
$parents = $statment->fetchAll();
foreach ($parents as $value) {
if ($value['id'] == "Loay") {
throw new \Exception('Person can not be moved to its children.');
}
}
Note: Here I'm not aware of table structure completely. So just given a general example.
Update Query
$statment = $this->db->prepare("UPDATE person SET parent = :parentId WHERE id = :childId");
$statment->execute([":parentId" => $parentId, ":childId" => $childId]);
$result = $statment->rowCount();