If you check for ! $value in the validation in the example callback method slug_update_spaceship, a string value of '0' (or anything else that PHP evaluates to false) will be ignored and that post meta field will not be updated.
Since '0' might be a desired post meta value, I propose changing the validation on this line to instead check isset and ensure the string's length is >0.
If you check for
! $valuein the validation in the example callback methodslug_update_spaceship, a string value of'0'(or anything else that PHP evaluates to false) will be ignored and that post meta field will not be updated.Since
'0'might be a desired post meta value, I propose changing the validation on this line to instead checkissetand ensure the string's length is >0.