What is the significance of the HTTP_IF_MODIFIED_SINCE header in PHP?

The HTTP_IF_MODIFIED_SINCE header in PHP is used to determine if a resource has been modified since a specific date and time. This header is typically sent by the client to the server when requesting a resource. By checking this header in PHP, you can determine if the resource needs to be sent again or if the client can use a cached version. 

if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])){
    $if_modified_since = strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']);
    $last_modified = strtotime('last modified date of the resource');
    
    if($last_modified <= $if_modified_since){
        header("HTTP/1.1 304 Not Modified");
        exit;
    }
}

// Send the resource if it has been modified

Keywords

HTTP_IF_MODIFIED_SINCE header significance PHP

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