<?php
$subject = '[2017-03-14 11:48:22] Steve T: Hi!
[2017-03-14 11:49:01] Oscar: Hi! :D How are u doin?
[2017-03-14 11:50:24] Steve T: Im doing great :P';
$returnValue = preg_match_all(
'/^\\[(\\d{4}-\\d{2}-\\d{2} \\d{2}:\\d{2}:\\d{2})\\] ([\\w\\s]+): (.+)$/m',
$subject,
$matches,
PREG_SET_ORDER // this groups the result nicely
);
print_r($matches);