Here, first we will extract the data which we want to export.
Then, will make make headers for exported file and then append the exported data
then will forcefully download the file.

<?php
    /* * Export Mysql Data in excel or CSV format using PHP * Downloaded from http://beingidea.info */    
    // Connect to database server and select
    $con=mysqli_connect('localhost','root','','db');
    
    if (mysqli_connect_errno()) {
    echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }
    
    // retrive data which you want to export
    $query = "SELECT * FROM category limit 60";
    $header = '';
    $data ='';
    
    $export = mysqli_query($con,$query ) or die(mysqli_error($con));
    
    // extract the field names for header
    
    while ($fieldinfo=mysqli_fetch_field($export))
    {
    $header .= $fieldinfo->name."t";
    }
    
    // export data
    while( $row = mysqli_fetch_row( $export ) )
    {
    $line = '';
    foreach( $row as $value )
    { 
    if ( ( !isset( $value ) ) || ( $value == "" ) )
    {
    $value = "t";
    }
    else
    {
    $value = str_replace( '"' , '""' , $value );
    $value = '"' . $value . '"' . "t";
    }
    $line .= $value;
    }
    $data .= trim( $line ) . "n";
    }
    $data = str_replace( "r" , "" , $data );
    
    if ( $data == "" )
    {
    $data = "nNo Record(s) Found!n"; 
    }
    
    // allow exported file to download forcefully
    header("Content-type: application/octet-stream");
    header("Content-Disposition: attachment; filename=beingidea_com_export.xls");
    header("Pragma: no-cache");
    header("Expires: 0");
    print "$headern$data";
    ?>