Description:
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---
From manual page: http://www.php.net/mysqli-result.num-rows
---
it doesnt say what gets returned on error (for comparison mysql_num_rows, according to the docs throws false).

by thorwing in a value that cant work (for example a string), it returned NULL and a warning.

since both NULL and 0 are variables that are false-y and according to http://php.net/manual/de/types.comparisons.php#types.comparisions-loose both (NULL == 0) and (NULL == 0) evaluate to true there should be a warning box, noting that strict comparison using === should be appropriate, similar to http://php.net/manual/de/function.strpos.php

Test script:
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<?php
var_dump(mysqli_num_rows("abc"));