Php variable declaration with an interrogation point before the type

Question

In php, what is the difference between the two following variable declarations, both for procedural mode and inside a class:

• string $str; // without ? before the type
• ?string $str; // with ? before the type

Thank you for your lights

Answer 1

Variable $str can only be of type string:

string $str;

Variable $str can be of type string or null:

?string $str;