Add extra user registration fields in WordPress

# Add extra user registration fields

function czystespalanie__registration_fields() {

    # Get and set any values already sent
    $first_name = ( isset( $_POST['first_name'] ) ) ? $_POST['first_name'] : '';
    $last_name = ( isset( $_POST['last_name'] ) ) ? $_POST['last_name'] : '';
    $tel = ( isset( $_POST['tel'] ) ) ? $_POST['tel'] : '';

    # Simple spam bots and crawlers protection system

    $a = mt_rand( 1, 9 );
    $b = mt_rand( 1, 9 );

    if( mt_rand( 0, 1 ) === 1 ) {
        $sum = $a + $b;
        $sign = " + ";
    } elseif  ( ( $a - $b ) === 0 ) {
        $sum = ++$a - $b;
        $sign = " - ";
    } else {
        $sum = $a - $b;
        $sign = " - ";
    }

    ## Hash sum before send with POST

    $_pre_captcha = md5 ( $sum );

?>

<h4><?php _e( '*) Profile information', 'czystespalanie' ); ?></h4>
<hr style="margin-bottom: 5px;"/>
<p>
<label for="first_name"><?php _e( 'First name', 'czystespalanie' ) ?></label>
<input type="text" name="first_name" id="first_name" class="input" value="<?php echo esc_attr( stripslashes( $first_name ) ); ?>" required>

</p>
<p>
<label for="last_name"><?php _e( 'Surname', 'czystespalanie' ) ?></label>
<input type="text" name="last_name" id="last_name" class="input" value="<?php echo esc_attr( stripslashes( $last_name ) ); ?>" required>

</p>
<p>
<label for="tel"><?php _e( 'Telephon:', 'czystespalanie' ) ?></label>
<input type="tel" name="tel" id="tel" class="input" value="<?php echo esc_attr( stripslashes( $tel ) ); ?>" required>

</p>

<p><h4><?php _e( 'Anti-spam protection', 'czystespalanie' ); ?> <img class="anti-spam-shield" src="<?php echo get_template_directory_uri() . '/inc/images/anti-spam-protection.png'; ?>"></h4>
<hr style="margin-bottom: 5px;"/>
    <label for="captcha" class="col-sm-4 control-label">
    <?php _e( 'Solve the equation:', 'czystespalanie' ); echo ' ' . $a . $sign . $b; ?> = ?
    </label>
    <input type="hidden" name="czystespalanie__pre_captcha" value="<?php echo esc_attr( $_pre_captcha ); ?>">
    <input type="text" name="czystespalanie_captcha" class="form-control" id="captcha">
</p>
<p style="margin-bottom: 5px;"><small><?php _e( '*) No additional profile details are required. You can complete them later in the dashboard.', 'czystespalanie' ); ?></small></p>
<hr/>
    <?php
}
add_action( 'register_form', 'czystespalanie__registration_fields' );