What is the difference between “is_singular()” and “is_single()” ?

https://codex.www.ads-software.com/Conditional_Tags#A_Single_Post_Page

https://phpxref.com/xref/wordpress/_functions/is_single.html
is a function:
https://phpxref.com/xref/wordpress/wp-includes/query.php.source.html#l260

https://phpxref.com/xref/wordpress/_functions/is_singular.html lloks like a variable:
https://phpxref.com/xref/wordpress/wp-includes/query.php.source.html#l402

is_single() returns true if any single post is being displayed is_singular() returns true when any page, attachment, or single post is being displayed. I figured this out from searching the codex and a little of my own testing.

https://codex.www.ads-software.com/Function_Reference/is_single

https://codex.www.ads-software.com/Function_Reference/is_singular