function url_exists($url) {
    $ch = curl_init(); 
    curl_setopt ($ch, CURLOPT_URL, $url); 
    //不下载
    curl_setopt($ch, CURLOPT_NOBODY, 1);
    //设置超时
    curl_setopt ($ch, CURLOPT_CONNECTTIMEOUT, 3); 
    curl_setopt($ch, CURLOPT_TIMEOUT, 3);
    curl_exec($ch);
    $http_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);  
    if($http_code == 200) {
        return true;
    }
    return false;
}

$url = 'http://www.phpddt.com/img/qrcode_for_phpddt.JPG';
if(@fopen($url, 'r')) {
    echo '文件存在';
} else {
    echo '文件不存在';
}

$url = 'http://www.phpddt.com/img/qrcode_for_phpddt.JPG';
 
stream_context_set_default(
    array(
        'http' => array(
             'timeout' => 1,
            )
    )
);
 
$headers = get_headers($url);
 
if(preg_match('/200/',$headers[0])) {
    echo '文件存在';
} else {
    echo '文件不存在';
}

 $opts = array(
    'http'=>array(
    'timeout'=>3,
    )
);
$context = stream_context_create($opts);
$resource = @file_get_contents('http://www.phpddt.com/img/qrcode_for_phpddt.JPG', false, $context);
 
if($resource) {
    echo '文件存在';
} else {
    echo '文件不存在';
}