help me to change mysql code to php

There is no reason why you can't run this query as is:

PHP Code:

// In your model
$this->load->database();

$query = $this->db->query('
     SELECT a1.mark_id, a1.student_id,  
     COUNT(a2.quiz) Rank 
     FROM mark a1, mark a2 WHERE a1.quiz < a2.mark_quiz
     OR (a1.mark_total=a2.quiz AND a1.student_id = a2.student_id) 
     GROUP BY a1.student_id, a1.quiz ORDER BY a1.quiz DESC, a1.student_id DESC
');

if( $query->num_rows() > 0 )
{
     // Do something with $result ...
     $result = $query->result();
} 


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