$img_file='http://www.somedomain.com/someimage.jpg'

$img_file=file_get_contents($img_file);

$file_loc=$_SERVER['DOCUMENT_ROOT'].'/some_dir/test.jpg';

$file_handler=fopen($file_loc,'w');

if(fwrite($file_handler,$img_file)==false){
    echo 'error';
}

fclose($file_handler);

$content = file_get_contents('http://example.com/image.php');
file_put_contents('/my/folder/flower.jpg', $content);

参见file()PHP手册:

$url    = 'http://mixednews.ru/wp-content/uploads/2011/10/0ed9320413f3ba172471860e77b15587.jpg';
$img    = 'miki.png';
$file   = file($url);
$result = file_put_contents($img, $file)

$data = file_get_contents('http://example.com/image.php');
$img = imagecreatefromstring($data);
imagepng($img, 'test.png');

如果allow_url_fopen设置为true:

$url = 'http://example.com/image.php';
$img = '/my/folder/flower.gif';
file_put_contents($img, file_get_contents($url));

否则使用cURL:

$ch = curl_init('http://example.com/image.php');
$fp = fopen('/my/folder/flower.gif', 'wb');
curl_setopt($ch, CURLOPT_FILE, $fp);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_exec($ch);
curl_close($ch);
fclose($fp);

使用PHP的函数copy():

copy('http://example.com/image.php', 'local/folder/flower.jpg');

注意:这需要allow_url_fopen