How to give a download link to a .csv file from the custom plugin?

Question

I have made a custom plugin, which exports MYSQL table contents into a .csv file which gets saved in to wp uploads directory. Now, I am trying to give a download link for downloading directly a .csv file. But, I am getting this error "Warning: Cannot modify header information - headers already sent"

I'm so very grateful for all of the help you all give. I've searched for this topic and couldn't find it, if this topic is already answered here, please comment with the link to the question with the answer.

Thanks.

Code Which I tried for Downloaing:

header( "Cache-Control: must-revalidate, post-check=0, pre-check=0" );
header( "Content-Length: " . strlen( $out ) );

// Output to browser with appropriate mime type, you choose ;)
header( "Content-type: text/x-csv" );
header( "Content-Disposition: attachment; filename=$filename" );

echo $out;
exit;

Answer 1

In your plugin main file eg.... my-plugin-def.php

Add the following lines

add_action( 'admin_post_sample_data.csv', 'sample_data_csv' );

function sample_data_csv()
{
    if ( ! current_user_can( 'manage_options' ) )
        return;

    header('Content-Type: application/csv');
    header('Content-Disposition: attachment; filename=example.csv');
    header('Pragma: no-cache');

    // output the CSV data
    echo 'id,name';
    echo '1,Saran';
}

Where you required download link add this line

echo '<a href="' . admin_url( 'admin-post.php?action=sample_data.csv' ) . '">Download sample</a>';