My approach to dealing with this issue is to use the Client URL Library (cURL) (English) to collect HTTP Status code (English) and with the whether or not the image exists at the given URL:

/**
 * URL Exists
 *
 * Verifica se o caminho URL existe.
 * Isso é útil para verificar se um arquivo de imagem num 
 * servidor remoto antes de definir um link para o mesmo.
 *
 * @param string $url           O URL a verificar.
 *
 * @return boolean
 */
function url_exists($url) {

    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_NOBODY, true);
    curl_exec($ch);
    $code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
    curl_close($ch);

    return ($code == 200); // verifica se recebe "status OK"
}

Usage example:

$url = 'http://www.example.com/minha_imagem.jpg';
if (url_exists($url))
    // encontrei :)
else
    // não encontrei :(

Note: This solution depends on functions added from PHP 4.0.2.

For this you use file_exists , in addition to remote URL it works with absolute or relative paths of the server itself, like this:

    if(file_exists('http://www.dominio.com/imagens/minha-imagem.jpg')){
       //seu código...
    }

Other useful functions are:

• is_readable - Says whether the file can be read.
• file - Reads the entire file to an array
• file_get_contents - Reads the whole file to a string (extremely useful)
• fread - Binary-safe file reading
• readfile - Reads and displays the contents of a file

I've set up the function below that can check both images and web pages, or any other content that comes from the web. I get the header response from the request and compare the codes. If it is 200 or 302 exists; if it is 500 has an internal error on the server; if it is 404 does not exist.

It is worth remembering that the one who will make the request is your web server, and the same should have access to the internet.

<?php
function validarext($url)
{
    $validar = get_headers($url);
    $validar = explode(" ",$validar[0]);
    $validar = $validar[1];
    if($validar == "302" || $validar == "200")
        return true;
    else
        return false;
}

if(validarext("http://www.urldaimagem.com.br/imagem.jpg")){
    //Imagem existe
    echo "Sim";
}
else
{
    //Imagem não existe...
    echo "Não";
}
?>

In the system that implements the fileUpload you do a routine that will search if a certain photo already exists. This function will be useful for all processes:

1st: As said, check if the photo exists before creating the links to it, how to do is using the implementation of file_exists () ;. It is recommended that you use a database to store the name of your file so you can request it and check with the absolute path of your server.

2nd: When you verify that the image exists, and then you are told to execute the replacement process, you will delete the existing image using the unlink () function; and then retrieve the new ~ temp image and rename it to the name that is in the database.

3rd: Make an implementation of a hash pattern to append to your files, every time someone uploads a file, get the name of the file and concatenate it with the following statement uniquid (rand (6, 8));

<?php

$nome = $_FILE['image']['name'];
$novoNome = $nome . uniquid( rand(6, 8) );