“Cannot modify header information – headers already sent by …”

How can we detect whether the script has already sent out headers? PHP has a function headers_sent() which allows you to check if the headers are already sent out before you take any action. Here’s how you could use the function in your code:

if(headers_sent())
{ //if headers already sent out print some message.
echo "Please go to yahoo.com";
}
else{
//send the user automatically to test.php
header('Location: http://yahoo.com');
}

The code above checks to see if PHP has already sent out headers anywhere else. If so, the script will print a message on screen, else the user is automatically redirected to the yahoo.com.