Links

Check out The Definitive Guide To PHP’s isset() And empty() for a deep dive on these faux-function language constructs.

TL;DR

• always develop with error reporting turned to 11 (and fix all real errors)
• always use empty or isset for variables that may legitimately not exist
• never use empty or isset for variables that should exist

Note: To deal with PHP notices in 3rd-party code, if you can’t contribute fixes upstream you may want to consider using a plugin like Zack Tollman’s Ostrichcize.

it [is] impossible to distinguish between a variable that does not exist and a variable that has the value null. […] Specifically for arrays, there’s an alternative to isset($array['key']) called array_key_exists. This function specifically does what it says: it returns true if an array key exists, regardless of the value of that key. That makes it possible to detect null values in arrays

Note that array_key_exists() is used in WP_Customize_Manager::post_value() for this reason. Since the Customizer’s dirty setting values are encoded as JSON it is possible for a post value to be null (although wp.customize.Value doesn’t directly facilitate seting this value).

/via @Josh412