php Function returns zero instead of the expected string

Question

In my example I am collecting form data from a Gravity Forms radio button field (Field ID 4). Field ID 4 has a value of 0 for the selection of "First Choice". So to address the field I am using the variable $field_id and the selection value is gathered by using $entry[$field_id] (which is the value of 0) and the text label for this value is "First Choice". The expected result of field ID 4 with a value of 0 is the text "First Choice". I need this printed to a text file. I have done the following:

    $field_id = 4;
    $field = GFFormsModel::get_field( $form, $field_id );
    $choice_text = $field['choices'][$entry[$field_id]]['text'];

    $data = $choice_text;

    file_put_contents($my_file,$data);
    $tmpfile = fopen($my_file, "r");
    $contents = fread($tmpfile, filesize($my_file));

The above approach will print "First Choice" to my text file as expected. However I need to do this with a function. If I try to do this with the following function I am returning 0 for some reason:

    function choiceLabel($field_id){
        $field = GFFormsModel::get_field( $form, $field_id );
        $choice_text = $field['choices'][$entry[$field_id]]['text'];
        return $choice_text;
    }

    $data = choiceLabel(4);

    file_put_contents($my_file,$data);
    $tmpfile = fopen($my_file, "r");
    $contents = fread($tmpfile, filesize($my_file));

This produces an error that says "Warning: fread(): Length parameter must be greater than 0". Why is my function returning a 0 value instead of the string "First Choice"? Thanks for looking at this.

Answer 1

Thanks to David for pointing me in the direction of scope.

This works:

    function choiceLabel($field_id,$form,$entry){
        $field = GFFormsModel::get_field( $form, $field_id );
        $choice_text = $field['choices'][$entry[$field_id]]['text'];
        return $choice_text;
    }

    $data = choiceLabel(4,$form,$entry);