I have tried a few plugins and can’t seem to find one that works for a simple PHP include into my WordPress page. Does anybody have any recommendations? And is there a reason that WordPress doesn’t support PHP includes into pages via the WYSIWYG editor by default?
The WYSIWYG editor restricts the type of code you can use for security reasons.
You would have to add the PHP include directly to the code in the page template you want to alter. Go to Appearance > Editor to find the template.
Just be aware that if you change the code in a theme, it will be over-written the next time an update for the theme becomes available and you update it. You will lose all your changes then. If you want to change the theme’s code, you should really use a child theme.
The Page that I am posting to already uses a template. However, the SVG include needs to be within the content area of that template. So I believe I need to use the WYSIWYG.
Is there a PHP plugin that works with the latest version of WordPress for this?
Could you make the php code you want to include into a template? Then this plugin: https://wordpress.org/plugins/improved-include-page/ might do the job for you. It seems to include a template inside another page or post.