After migrating my site to a new hosting service, I was getting a perplexing error, namely:
“Warning: call_user_func_array() expects parameter 1 to be a valid callback, no array or string given… in class-wp-hook.php”
Searching, there were lots of discussion about hooks and functions and arrays, but nothing definite.
At the same time, I also noticed that my ‘glyphs’ or symbols were messed up – for example, > was showing as 5, the search icon showed as a U.
Clearly, it was all related to the migration. Was I missing some files?
A good starting point when looking for WordPress problems is the browser Console – I used the Inspect tool in Chrome to view the console, and immediately saw what could have been the issue – certainly, the Glyphs weren’t showing because there was a cross-domain font issue: I had transgressed the “Cross-Origin Resource Sharing policy”!
In other words, I had a typo in my WordPress domain name. A fat-fingered extra full-stop in my domain name had caused both the font issue and the WordPress error…