don't forget to add in the bearer token on line 23

slack-afk-automator.php

<?php

function set_myself_afk() {
	$tz = date('I') ? 'EST' : 'EDT';
	$expiration = strtotime( "+1 Weekday 9am {$tz}" );

	$weekday = array(
		'status_text'       => 'AFK for the evening.',
		'status_emoji'      => ':family:',
		'status_expiration' => $expiration,
	);

	$weekend = array(
		'status_text'       => 'AFK for the weekend.',
		'status_emoji'      => ':weekend-2:',
		'status_expiration' => $expiration,
	);

	$opts = array(
		'http' => array(
			'method' => 'POST',
			'header' => array(
				'Authorization: Bearer TOKEN_GOES_HERE',
				'Content-Type: application/json',
			),
			'content' => json_encode(
				array(
					'profile' => ( 5 <= (int) date( 'w' ) ) ? $weekend : $weekday,
				)
			),
		),
	);

	$contents = file_get_contents(
		'https://slack.com/api/users.profile.set',
		false,
		stream_context_create( $opts )
	);

	return $contents;
}

set_myself_afk();

The crontab entry I use to fire this at 3:30pm on Weekdays for me is --

30 15 * * 1-5 /usr/local/bin/php /the/path/to/slack-afk-automator.php >/dev/null 2>&1