Description:
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the current HashContext only has a procedural interface, 
some people would prefer having an Object-oriented interface instead. (the same way mysqli_query() & co has both a procedural and object-oriented api)

for example, the current way to hash $str1 and $str2 is:

$hc = hash_init("SHA1");
hash_update($hc, $str1);
hash_update($hc, $str2);
$result = hash_final($hc);

(i've omitted error-checking for brevity)
but if we had an OO interface, it could also be written as: 

$result = (new HashContext("SHA1"))->update($str1)->update($str2)->final();

some people would prefer that. something like this: https://3v4l.org/lXd3u


Test script:
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<?php
$str1 = "foo";
$str2 = "bar";
$result = (new HashContext("SHA1"))->update($str1)->update($str2)->final();
echo $result;

Expected result:
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8843d7f92416211de9ebb963ff4ce28125932878