mail not getting sent on server - php

Question

I have 2 websites on hosted but on one web site I can send mail using php script using other web site I cannot mail using the php script. I don't know where I am going wrong. I even replaced php.ini file. Is there any other thing which I left doing or something.
This is my code
$to="abc#gmail.com";
$subject="test mail";
$body="test mail";
$headers = "MIME-Version: 1.0" . "\r\n";
$headers .= "Content-type:text/html;charset=iso-8859-1" . "\r\n";
$headers.='From: Domain <def#gmail.com>' . "\r\n";
if(mail($to,$subject,$body,$header))
{
echo "mailed";
}
else
{
echo "not mailed";
}
I even tried directly embedding message into the mail function that also doesn't seems to be working.
This is the code after embedding message
if(mail("abc#gmail.com", "Test subject", "Test Message", "From: def#gmail.com"))
{
echo "mailed";
}
else
{
echo "error:".mysql_error();
}
I tried contacting my hosting providers technical people. Even they could not find the solution.

Answer 1

it's Sample And Works for me:
$to ='Info#sample.com';
$message = '<a>Hellow</a>';
$subject ='Hello world';
$headers = "MIME-Version: 1.0" . "\r\n";
$headers .= "Content-type:text/html;charset=UTF-8" . "\r\n";
$headers .= "From: <sender#email.com>" . "\r\n";
$mail_sent=#mail($to,$subject,$message,$headers);
echo $mail_sent ? "Success" : "Error";
if this code not working , Call to Your server Administrator For check Server...