The opening tag
<?php is mandatory. The closing one
But it is recommended not have the closing tag at all, as it can break/freeze the site due to extra white spaces after it.
It's also recommended not to use the short opening tag
<?, as some servers may not have it enabled.
Then you may have some Html being printed in the middle of PHP code, when the later is closed and reopened after the former ended.
Or vice versa, when the PHP file starts with Html, and the PHP will be opened-closed and Html follows.
Documentation (as usual there, the comments are worth reading too):
In this Question's context, if the parent's
functions.php has a closing tag, just check if no white spaces after it, or simply remove it.
And in the child's
functions.php, just put the opening tag and start writing/pasting all the needed mods/enhancements. Taking care that extra
<?php ?> are under control.