感谢您的答复.我使用它来工作:

Thanks for the response. I got it to work using:

- (IBAction) login: (id) sender
{

NSString *post =[NSString stringWithFormat:@"username=%@&password=%@",usernameField.text, passwordField.text];

NSString *hostStr = @"MY URL.php?";
hostStr = [hostStr stringByAppendingString:post];
NSData *dataURL =  [NSData dataWithContentsOfURL: [ NSURL URLWithString: hostStr ]];    
NSString *serverOutput = [[NSString alloc] initWithData:dataURL encoding: NSASCIIStringEncoding];
if([serverOutput isEqualToString:@"Yes"]){
    UIAlertView *alertsuccess = [[UIAlertView alloc] initWithTitle:@"success" message:@"You are authorized"
                                                          delegate:self cancelButtonTitle:@"No" otherButtonTitles:@"Yes", nil];
    [alertsuccess show];
    [alertsuccess release];

} else {
    UIAlertView *alertsuccess = [[UIAlertView alloc] initWithTitle:@"Fail" message:@"Invalid Access"
                                                          delegate:self cancelButtonTitle:@"No" otherButtonTitles:@"Yes", nil];
    [alertsuccess show];
    [alertsuccess release];

}

loginIndicator.hidden = FALSE;
[loginIndicator startAnimating];
loginButton.enabled = FALSE;
}