One of the powers of MODX is its flexible templating. The biggest part of the flexibility boils down to the different types of tags that can be used in Templates which replaced with the dynamic content specified.
While the different tags are listed in the Documentation, not all of them are properly explained and some take time to really make sense when you just started using MODX. This blog post will explain the difference between two tags that are very often confused with eachother: placeholder and resource field tags.
Let's see what the two terms actually mean and which is which.
A resource field is a field of a resource. Oh, you figured that out already? Great! So if you fill in fields of a certain resource in the Manager, the value of that field is available through the resource field tag. What is important to realize here is that you can use these tags, no matter where they are used (in templates, chunks or elsewhere), will take the value for the current resource. So if you're viewing the homepage, a resource field tag will get the value from the homepage resource. If you're looking at a contact page, the resource field tag will be replaced with the values of the contact page.
It may seem logical or common sense (and really, it is!), but that's where placeholders differ from resource fields.
Resource fields use the *asterix* token in the tag. Some valid examples:
And their values for the current resource:
Title: February 2012
Menu Index: 1
Content (not shown to prevent the page from breaking!)
A place holder can be anything.
Yes, really. Anything.
The key thing to realize with placeholders is that they are being specifically set by usually a snippet or plugin. That snippet or plugin decides what a specific placeholder will get as a value. Some snippets such as getResources or my getRelated iterate over resources, and they assign placeholders with the same name as the resource field.
Let's go a bit more practical.. You may have a resource listing on your homepage (for example the latest Articles), retrieved with the getResources snippet. Your Homepage template will probably use the Resource Field syntax to display the current page's title in the <head> part:
Your getResources snippet on the other hand uses placeholders to provide you with information on the resource it is looking at. With getResources you specify the &tpl property which is the name of a chunk that will be used when iterating over resources. The chunk may look like this:
This means that when getResources reads the resources with ID 2, 3, 4 and 5, it will load the tpl chunk four times, and assigns values as placeholders each of those four times. The first time it loads the chunk it sets the values to the resource fields of resource 2. The second time it takes the resource field values for resource 3, the third time for resource 4 and finally for resource 5. So in this case we have placeholders that give information on a resource which is not the current resource.
Resource Fields always contain values for the current resource, while placeholder values can be anything from other resources, custom tables or simply random stuff.
A note about Placeholders
While placeholders are usually set by snippets or plugins, that doesn't mean that's the only way they can be set or used.
You can also add parameters to a chunk tag, which can then be used as placeholders within a chunk. This is a nifty technique if you only need minor changes depending on the template, and want to prevent repeating yourself.