Filter users by a custom field? From within a plugin, I’m trying to filter users by a custom field. 

An external post request will supply a variable which I want to match the custom field to. I then want to respond to that request with other user custom fields as JSON. 

For example the incoming post contains customHandle1=A, I then find all the users where customHandle1=A and return customHandle2 and customHandle3 data.  

How would I do that?
To get a user by a custom field, you would need to create a ElementCriteriaModel like this:

$criteria = craft()-&gt;elements-&gt;getCriteria(ElementType::User);
$criteria-&gt;fieldName = craft()-&gt;request-&gt;getPost('fieldName');

$users = $criteria-&gt;find();


you could then loop over $users and use getAttributes() to get the user data, and if you need the attribute names use attributeNames() if you need the users fields.