Syslog autodetects newline control characters and therefore splits the message by multiple lines. To prevent this behavior in PHP 7.3+ you can use undocumented (at this moment) ini setting:
<?php
ini_set('syslog.filter', 'raw');
# more info here: https://bugs.php.net/bug.php?id=77913