more examples: https://issues.joomla.org/tracker/joomla-cms/41803#event-756087

Unable to replicate with php 8.1.10

perhaps this has been solved elsewhere?

Closing as non-reproducible.

Code has not been changed (as debugged above where the root cause is).
so here you go again with php 8.2.15 in this case

Please advise exact steps in which file as I cannot reproduce it. Which Joomla version? Thanks.

Latest Joomla 5 version.

I have done a root cause analysis and proposed a fix when you do a code review for this it should be clear:

Trim requires a string as input.
The result of the sql field is mixed: string when a value is goud or null when there is no value found. The null (empty sql result) is then passed to the trim function that will then throw this error.

No need to replicate

Question here is how do we handle 'user' input?
In this case we trust the sql field to always return a value.... Which it doesn't

So we either force the input to be a string via casting (that is what I did in proposed solution), or do not accept other value then strings.

There are more places where these situations can occur in core code: assumptions being made of the received input being correct

I have already linked to an example in previous comment.

unable to replicate
this query have 0 results

<field
    name="created_by"
    type="sql"
    sql_select="a.id, a.created_by, u.id, u.name AS name"
    sql_from="#__content AS a"
    sql_join="#__users AS u ON a.created_by = u.id"
    sql_where="u.id = 0"
    sql_order="u.name ASC"
    key_field="created_by"
    value_field="name"
    onchange="this.form.submit();"
    >
    <option value="">COM_EXAMPLE_CLICKEDBY_FILTER</option>
</field>

@alikon it is PHP issue, when there is a call of select.option with null instead of expected string, like:

HTMLHelper::_('select.option', 'foobar', null);

Not that it is correct call, but it is what it is

Did some more debugging, issue arises when SQL field holds value NULL.

in administrator/components/com_content/forms/filter_articles.xml

add the following filter field:

<field
    name="created_by"
    type="sql"
    sql_select="a.id, a.created_by, u.id, u.lastResetTime AS name"
    sql_from="#__content AS a"
    sql_join="#__users AS u ON a.created_by = u.id"
    sql_where="u.id != 0"
    sql_order="u.name ASC"
    key_field="created_by"
    value_field="name"
    onchange="this.form.submit();"
    >
			<option value="">COM_OCHCLICKTHIS_CLICKEDLINKS_LINK_FILTER</option>
</field>

The problem isn't so much the class you mentioned, but the classes using this class. They are handing in empty entries, which is triggering this error.

Hi @Hackwar thanks for following up and agree. But long time ago I have learned never to trust input passed by a user. That should always be checked. So instead of not checking and in this case throw a warning (which might turn into an error in future PHP versions) It should IMO check and handle the error.
The thing here is that the data comes from another Joomla core method that can correctly output null as value (nothing selected).
So when the class using this class should be changed then the sql field should be changed e.g. by having a default value when the query result is null