未定义的索引：HTTP_USER_AGENT

I am trying to perform an action when a specific user agent visits a link.

So i have my code like this:

//if browser is not Mozilla/4.2, then do something. 
//but if its Mozilla/4.2, do another thing.

if(strlen(strstr($_SERVER['HTTP_USER_AGENT'],"Mozilla/4.2")) <= 0 ){ 
    // Do something
} else {
    //Else do another thing code follows.
}

The above code is working but it keeps giving this warning in the error log "Undefined index: HTTP_USER_AGENT"

The solution i saw used pregmatch, but am targeting only a single user agent.

Any help will be appreciated.

You simply need to check the existence of the index (HTTP_USER_AGENT) on $_SERVER and if it's not set then set it to empty string.

This can be achieved by doing;

$userAgent = ! empty($_SERVER['HTTP_USER_AGENT']) ? $_SERVER['HTTP_USER_AGENT'] : '';

if(strlen(strstr($userAgent,"Mozilla/4.2")) <= 0 ) { 
    // Do something
} else { 
    // Do something else.
}