PHP - Tabla con Json

<?php

include('conexion.php');

$sql = "SELECT m.materia, m1.materia, c.carrera, '' as correlativafinal, idcorrelativa FROM correlatividades ca, materia m, materia m1, carrera c WHERE m.idmateria = ca.materia and ca.materiacorrelativa = m1.idmateria and c.idcarrera = ca.carrera ";

$sql .= "UNION ";

$sql .= "SELECT m.materia, '' as materia, c.carrera, m1.materia, idcorrelativa FROM correlatividades ca, materia m, materia m1, carrera c WHERE m.idmateria = ca.materia and ca.correlativafinal = m1.idmateria and c.idcarrera = ca.carrera";

$rs = mysql_query($sql,$coneccion);

$cantidad = mysql_num_rows($rs);

$i=0;

$arr = '';

while($row=mysql_fetch_array($rs)){

	$i = $i + 1;

	$arr .= '["'.$row[0].'","'.$row[1].'","'.$row[2].'","'.$row[3].'",';

	if($cantidad==$i){

		$arr .= '"<input type=\"image\" src=\"images/modificar.png\" width=\"16px\" name=\"mcorrelativa\" value=\"Modificar\" class=\"clscorrelativa\" id=\"m'.$row[4].'\">",';

		$arr .= '"<input type=\"image\" src=\"images/eliminar.png\" width=\"16px\" name=\"emateria\" value=\"Eliminar\" class=\"clscorrelativa\" id=\"e'.$row[4].'\">"]';

	}else{

		$arr .= '"<input type=\"image\" src=\"images/modificar.png\" width=\"16px\" name=\"mcorrelativa\" value=\"Modificar\" class=\"clscorrelativa\" id=\"m'.$row[4].'\">",';

		$arr .= '"<input type=\"image\" src=\"images/eliminar.png\" width=\"16px\" name=\"ecorrelativa\" value=\"Eliminar\" class=\"clscorrelativa\" id=\"e'.$row[4].'\">"],';

	}

}

$arr = '{"aaData":['.$arr.']}';

echo $arr;

?>