PHP：json_encode（）不起作用

Why my code doesn´t works?. I have another one with the same code and it works!. I dont know what´s the problem with this one.

<?php
include("../conexion.php");
if($_SERVER['REQUEST_METHOD'] == 'GET')
{
    $query = 'SELECT Id, Nombre, Descripcion FROM empresa ORDER BY Nombre';
    $sql = mysql_query($query, $link);
    mysql_close($link);
    while($row = mysql_fetch_array($sql, MYSQL_ASSOC)){
        $myEnterprises[] = array(
        'Id' => $row['Id'],
        'Nombre' => $row['Nombre'],
        'Descripcion' => $row['Descripcion']);
    }
    $myData = array('myEnterprises' => $myEnterprises);
    echo json_encode($myData);
}
?>

(PHP 5 >= 5.2.0, PECL json >= 1.2.0)
json_encode — Returns the JSON representation of a value

This is the minimum requirement for running json_encode() in your server. Check your PHP version. If the PHP version is less than 5.2.0 ,follow this link for implementation of json_encode in your server / php application .