Checkboxes in file user.php

[biologisch]

Hi!
I would like to change the monitor select list with checkboxes. So I found this

<option value="<?= $monitor['Id'] ?>"<?php if ( array_key_exists( $monitor['Id'], $monitorIds ) ) { ?> selected="selected"<?php } ?>><?= htmlentities($monitor['Name']) ?></option>

and changed it with that

<input type="checkbox" name="monitorIds[]" value="<?= $monitor['Id'] ?>"<?php if ( array_key_exists( $monitor['Id'], $monitorIds ) ) { ?> checked="checked"<?php } ?>>&nbsp;<?= htmlentities($monitor['Name']) ?><br>

And I comment out

<select name="monitorIds" size="4" multiple="multiple">

and

</select>

Is there a php guru who can help me please ?