<?php
function resize_image($file, $max_resolution){

    if(file_exists($file)){
        $original_image = imagecreatefromjpeg($file);

        //resolution
        $original_width = imagesx($original_image);
        $original_height = imagesy($original_image);

        //try width first
        $ratio = $max_resolution / $original_width;
        $new_width = $max_resolution;
        $new_height = $original_height * $ratio;

        if($new_height > $max_resolution) {
            $ratio = $max_resolution / $original_height;
            $new_height = $max_resolution;
            $new_width = $orignial_width * ratio;
        }
            if($original_image){
 $new_image = imagecreatetruecolor($new_width, $new_height);
imagecopyresampled($new_image,$original_image,0,0,0,0,$new_width,$new_height,$original_width,$original_height);
        imagejpeg($new_image, $file, 90);


        }

    }

}
    if(isset($_FILES['image']) && $_FILES['image']['type'] == 'image/jpeg'){

        move_uploaded_file($_FILES['image']['tmp_name'],$_FILES['image']['name']);
        $file = $_FILES['image']['name'];
        resize_image($file, "500");

        echo "<img src='$file'/>";
    }
?>

<form method="post" enctype='multipart/form-data'>
<input type="file" name="image"><br/>
<input type="submit" value="post">
</form>