Difference between `is_serialized_string` and `is_serialized`

If you check the source code of is_serialized() and is_serialized_string(), the difference will become clear.

is_serialized() checks if the data is serialized, whereas is_serialized_string() checks, if the serialized data is of type string.

var_dump( is_serialized(        serialize(NULL) ) ); // true
var_dump( is_serialized_string( serialize(NULL) ) ); // false

var_dump( is_serialized(        serialize(array(1,2,3)) ) ); // true
var_dump( is_serialized_string( serialize(array(1,2,3)) ) ); // false

var_dump( is_serialized(        serialize("hello") ) ); // true
var_dump( is_serialized_string( serialize("hello") ) ); // true

(fiddle)