如何使用PHP调整dir中的图像大小？

This is my code, for resize test/15378596684270_1.jpg from 136x136px to 48x48px but when test code , its' not resize any image how can i do ?

<?PHP
session_start();
include("connect.php");
$file = "test/15378596684270_1.jpg";
list($width, $height) = getimagesize($file);    
$newwidth = "48";
$newheight = "48";          
$image_info = getimagesize($file);  
if ($image_info['mime'] == 'image/jpeg') { 
    $src = imagecreatefromjpeg($file);
} elseif ($image_info['mime'] == 'image/gif') {
    $src = imagecreatefromgif($file);
} elseif ($image_info['mime'] == 'image/png') {
    $src = imagecreatefrompng($file);
}       
$tgt = imagecreatetruecolor(48, 48);
imagecopyresampled($tgt, $src, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
?>