file_get_contents when url doesn't exist

You need to check the HTTP response code:

function get_http_response_code($url) {
    $headers = get_headers($url);
    return substr($headers[0], 9, 3);
}
if(get_http_response_code('http://somenotrealurl.com/notrealpage') != "200"){
    echo "error";
}else{
    file_get_contents('http://somenotrealurl.com/notrealpage');
}

With such commands in PHP, you can prefix them with an @ to suppress such warnings.

@file_get_contents('http://somenotrealurl.com/notrealpage');

file_get_contents() returns FALSE if a failure occurs, so if you check the returned result against that then you can handle the failure

$pageDocument = @file_get_contents('http://somenotrealurl.com/notrealpage');

if ($pageDocument === false) {
    // Handle error
}

Each time you call file_get_contents with an http wrapper, a variable in local scope is created: $http_response_header

This variable contains all HTTP headers. This method is better over get_headers() function since only one request is executed.

Note: 2 different requests can end differently. For example, get_headers() will return 503 and file_get_contents() would return 200. And you would get proper output but would not use it due to 503 error in get_headers() call.

function getUrl($url) {
    $content = file_get_contents($url);
    // you can add some code to extract/parse response number from first header. 
    // For example from "HTTP/1.1 200 OK" string.
    return array(
            'headers' => $http_response_header,
            'content' => $content
        );
}

// Handle 40x and 50x errors
$response = getUrl("http://example.com/secret-message");
if ($response['content'] === FALSE)
    echo $response['headers'][0];   // HTTP/1.1 401 Unauthorized
else
    echo $response['content'];

This aproach also alows you to have track of few request headers stored in different variables since if you use file_get_contents() $http_response_header is overwritten in local scope.