WordPress at a glance
function is not described

PHPMailer\PHPMailer

SMTP::client_send() public WP 1.0

Send raw data to the server.

{} It's a method of the class: SMTP{}

No Hooks.

Return

Int|true|false. The number of bytes sent to the server or false on error

Usage

$SMTP = new SMTP();
$SMTP->client_send( $data, $command );
$data(string) (required)
The data to send
$command(string)
Optionally, the command this is part of, used only for controlling debug output

Code of SMTP::client_send() WP 5.7.2

<?php
public function client_send($data, $command = '')
{
    //If SMTP transcripts are left enabled, or debug output is posted online
    //it can leak credentials, so hide credentials in all but lowest level
    if (
        self::DEBUG_LOWLEVEL > $this->do_debug &&
        in_array($command, ['User & Password', 'Username', 'Password'], true)
    ) {
        $this->edebug('CLIENT -> SERVER: [credentials hidden]', self::DEBUG_CLIENT);
    } else {
        $this->edebug('CLIENT -> SERVER: ' . $data, self::DEBUG_CLIENT);
    }
    set_error_handler([$this, 'errorHandler']);
    $result = fwrite($this->smtp_conn, $data);
    restore_error_handler();

    return $result;
}