How can I convert all images to jpg?

I have script:

<?php

include('db.php');
session_start();
$session_id = '1'; // User session id
$path = "uploads/";

$valid_formats = array("jpg", "png", "gif", "bmp", "jpeg");
if (isset($_POST) and $_SERVER['REQUEST_METHOD'] == "POST") {
    $name = $_FILES['photoimg']['name'];
    $size = $_FILES['photoimg']['size'];
    if (strlen($name)) {
        list($txt, $ext) = explode(".", $name);
        if (in_array($ext, $valid_formats)) {
            if ($size < (1024 * 1024)) { // Image size max 1 MB
                $actual_image_name = time() . $session_id . "." . $ext;
                $tmp = $_FILES['photoimg']['tmp_name'];
                if (move_uploaded_file($tmp, $path . $actual_image_name)) {
                    mysql_query("UPDATE users SET profile_image='$actual_image_name' WHERE uid='$session_id'");
                    echo "<img src='uploads/" . $actual_image_name . "' class='preview'>";
                } else {
                    echo "failed";
                }
            } else {
                echo "Image file size max 1 MB";
            }
        } else {
            echo "Invalid file format..";
        }
    } else {
        echo "Please select image..!";
    }
    exit;
}

?>

Is possible convert all images (png, gif etc) to jpg with 100% quality? If yes, how? I would like allow to upload png and gif, but this script should convert this files to jpg. Is possible this with PHP?

Try this code: originalImage is the path of... the original image... outputImage is self explaining enough. Quality is a number from 0 to 100 setting the output jpg quality (0 - worst, 100 - best)

function convertImage($originalImage, $outputImage, $quality)
{
    // jpg, png, gif or bmp?
    $exploded = explode('.',$originalImage);
    $ext = $exploded[count($exploded) - 1]; 

    if (preg_match('/jpg|jpeg/i',$ext))
        $imageTmp=imagecreatefromjpeg($originalImage);
    else if (preg_match('/png/i',$ext))
        $imageTmp=imagecreatefrompng($originalImage);
    else if (preg_match('/gif/i',$ext))
        $imageTmp=imagecreatefromgif($originalImage);
    else if (preg_match('/bmp/i',$ext))
        $imageTmp=imagecreatefrombmp($originalImage);
    else
        return 0;

    // quality is a value from 0 (worst) to 100 (best)
    imagejpeg($imageTmp, $outputImage, $quality);
    imagedestroy($imageTmp);

    return 1;
}

Try using Imagick setImageFormat, for me it provides the best image quality
http://php.net/manual/en/imagick.setimageformat.php

$im = new imagick($image);

// convert to png
$im->setImageFormat('png');

//write image on server
$im->writeImage($image .".png");
$im->clear();
$im->destroy(); 

Davide Berra's answer is great, so I improved the file type detection a little, using exif_imagetype() instead of relying on the file extension:

/**
*   Auxiliar function to convert images to JPG
*/
function convertImage($originalImage, $outputImage, $quality) {

    switch (exif_imagetype($originalImage)) {
        case IMAGETYPE_PNG:
            $imageTmp=imagecreatefrompng($originalImage);
            break;
        case IMAGETYPE_JPEG:
            $imageTmp=imagecreatefromjpeg($originalImage);
            break;
        case IMAGETYPE_GIF:
            $imageTmp=imagecreatefromgif($originalImage);
            break;
        case IMAGETYPE_BMP:
            $imageTmp=imagecreatefrombmp($originalImage);
            break;
        // Defaults to JPG
        default:
            $imageTmp=imagecreatefromjpeg($originalImage);
            break;
    }

    // quality is a value from 0 (worst) to 100 (best)
    imagejpeg($imageTmp, $outputImage, $quality);
    imagedestroy($imageTmp);

    return 1;
}

You must have php_exif extension enabled to use this.

A small code to convert image.png to image.jpg at desired image quality:

<?php
$image = imagecreatefrompng('image.png');
imagejpeg($image, 'image.jpg', 70); // 0 = worst / smaller file, 100 = better / bigger file 
imagedestroy($image);
?>