The expression $filter of type null|string is loosely compared to true; this is ambiguous if the string can be empty. You might want to explicitly use !== null instead.
In PHP, under loose comparison (like ==, or !=, or switch conditions),
values of different types might be equal.
For string values, the empty string '' is a special case, in particular
the following results might be unexpected:
''==false// true''==null// true'ab'==false// false'ab'==null// false// It is often better to use strict comparison''===false// false''===null// false
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In PHP, under loose comparison (like
==, or!=, orswitchconditions), values of different types might be equal.For
stringvalues, the empty string''is a special case, in particular the following results might be unexpected: