Description:
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The behavior of substr function have been changed on php 7, but not mention on the document.

Test script:
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<?php

$s = substr('str',3);
var_dump($s);


// result on php 5.x
// bool(false)

// result on php 7
// string(0) ""

Expected result:
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have the same result

Actual result:
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the return type not compatible.