我想出了如何继续。我用所需的选择选项创建了一个文本文件，并用下面的代码替换了模板中的输入字段。

<?php
    $file_path = \'FILE_PATH_HERE\'; // path to the file with select options
    if ( !file_exists( $file_path ) ) {
        echo \'<input type="text" name="location" placeholder="\' . __("Location ...", "custom-settings") . \'" value="\' . esc_attr($location) . \'" />\';
    } else {
        $file_array = str_replace("\\n","", file($file_path));
        $options = \'<option value="">\' . __("Select a location ...", "custom-settings") . \'</option>\';
        $options .= \'<option value="0">\' . __("All locations", "custom-settings") . \'</option>\';

        foreach ($file_array as $location)
            $options .= \'<option value="\' . $location . \'">\' . $location .\'</option>\';

        $select = \'<select id="location" name="location">\' . $options . \'</select>\';
        echo $select;
    }
?>