Filtering array of stdClass into single array

I want to be able to take an array of stdClass returned from PDO fetchAll and turn this into a single array.

So i get an array back that looks like this:

// print_r($getCategories); returns..

        Array
    (
        [0] => stdClass Object
            (
                [field1] => 'foo',
                [field2] => 'foo2'
            )

        [1] => stdClass Object
            (
                [field1] => 'bar'
                [field2] => 'bar2'
            )
    )

I loop through this and put a list of one field into a combo box

// <select> and <option> around this..

    foreach($getCategories as $category) {

        echo $category->field1;

    }

Now i know i could create an array by using the following code

foreach($getCategories as $category){

    if($_POST['category'] == $category->field1){

        $chosenCategory = array(

            'field1' => $category->field1,
            'field2' => $category->field2

        );

    }

}

I just want to know if there is a better way to do this where I don't have to build an array, instead just filter what is returned in $getCategories using user input?

I am fairly new to OOP and programming in general so an explanation as well as an answer would be much appreciated!

Thanks