Implementing RPDE

Always use a JSON library to generate the output, and never construct the JSON manually.
Language-specific examples of library implementations that also support the removal of nulls, empty strings and empty arrays - as the specification stipulates - are included here.
C# and .NET Framework
OpenActive.NET
We highly recommend using the OpenActive.NET library for .NET implementations. It includes strongly typed classes for the OpenActive data model including beta properties, and fully compliant serialisation methods too.
Manual Implementation
Use JSON.NET / Newtonsoft.Json with a ContractResolver to remove empty strings, and NullValueHandling to remove nulls. Empty list values must be removed by setting them to null in the generating code, aided by the extension ToListOrNullIfEmpty which is useful at the end of LINQ expressions.
public class RPDERestUtils
{
    public static HttpResponseMessage CreateJSONResponse(RpdePage page, HttpRequestMessage req)
    {
        var e = JsonConvert.SerializeObject(page,
            Newtonsoft.Json.Formatting.None,
            new JsonSerializerSettings
            {
                NullValueHandling = NullValueHandling.Ignore,
                ContractResolver = NoEmptyStringsContractResolver.Instance
            });
​
        var resp = req.CreateResponse(HttpStatusCode.OK);
        resp.Headers.CacheControl = new CacheControlHeaderValue()
        {
            Public = true,
            // Recommended cache settings from:
            // https://developer.openactive.io/publishing-data/data-feeds/scaling-feeds
            MaxAge = page?.items?.Count > 0 ? TimeSpan.FromHours(1) : TimeSpan.FromSeconds(8)
        };
        resp.Content = new StringContent(e, Encoding.UTF8, "application/json");
        return resp;
    }
​
    public class NoEmptyStringsContractResolver : DefaultContractResolver
    {
        public static readonly NoEmptyStringsContractResolver Instance = new NoEmptyStringsContractResolver();
​
        protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
        {
            JsonProperty property = base.CreateProperty(member, memberSerialization);
​
            if (property.PropertyType == typeof(string))
            {
                // Do not include empty strings in JSON output (as per OpenActive Modelling Specification)
                property.ShouldSerialize = instance =>
                {
                    return !string.IsNullOrWhiteSpace(instance.GetType().GetRuntimeProperty(member.Name).GetValue(instance, null) as string);
                };
            }
​
            return property;
        }
    }
​
    public static List<TSource> ToListOrNullIfEmpty<TSource>(this IEnumerable<TSource> source)
    {
        if (source != null && source.Count() > 0)
            return source.ToList();
        else
            return null;
    }
}
PHP
The specification requires that no null or empty strings are present in the OpenActive feed. To achieve this, define the RPDE response structure as nested arrays, and recursively unset empty properties before using json_encode to generate the response.
Run the example below here to see the result.
<?php
​
$testResponseStructure = array(
    "@context"=>"https://openactive.io/",
    "id"=>"https://example.com/api/sessions/1402CBP20150217",
    "identifier"=>"1402CBP20150217",
    "type"=>"SessionSeries",
    "organizer"=>array(
        "type"=>"Organization",
        "name"=>"Everyone Active",
        "url"=>null,
        "logo"=>array(
            "type"=>"ImageObject",
            "url"=>"",
        ),
        "email"=>null,
        "telephone"=> "01455 890508",
        "sameAs"=>array(
            null,
            ""
        ),
    ),
);
​
print json_encode_without_null_or_empty($testResponseStructure);
​
function json_encode_without_null_or_empty($nestedarr) {
    array_unset_recursive($nestedarr);
    return json_encode($nestedarr);
}
​
function array_unset_recursive(&$array) {
    foreach ($array as $key => &$value) {
        // First clean the array, in case it ends up empty
        if (is_array($value)) {
            array_unset_recursive($value, $remove);
            // Remove arrays containing only a "type" property
            if (count($value) == 1 && array_key_exists("type",$value))
                $value = null;
        }
​
        // Remove null values, empty strings, and empty lists
        if ($value !== false && ($value === null || $value === "" || empty($value))) 
            unset($array[$key]);
    }
}
Types of RDPE feed
Testing RPDE feeds
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