Storing a Specific Generated Random number as a Variable (PHP)

I'm trying to use random numbers to store the image names of uploaded images which works perfectly. Now, I have to use the number in another function, but it changes when used in another function.

$GLOBALS['rand'] = rand() . '.png';  

if(isset($_POST["image"])){
    $data = $_POST["image"];
    $image_array_1 = explode(";", $data);
    $image_array_2 = explode(",", $image_array_1[1]);
    $data = base64_decode($image_array_2[1]);
    file_put_contents( $GLOBALS['rand'], $data);
    echo '<img src="includes/'.$GLOBALS['rand'].'" class="img-thumbnail" />';
}

This is to display the cropped image and give it a name, before upload. Now to upload:

if(isset($_POST["submit"])){
    $pic = $GLOBALS['rand']; 
    $sql = "INSERT INTO users (pic) VALUES ('$pic');
}

I expect that when using the $GLOBALS['rand'] in the second function, the generated number is the same.