需要解释PHP中的＆运算符？ [重复]

Question

This question already has an answer here:

• Understanding PHP & (ampersand, bitwise and) operator 5 answers

i'm new here and i have a question that i couldn't find an answer to:

<?php 
$val1 = 25; 
$val2 = 15; 
echo ($value & $value1); // output : 9
?>

Can anyone explain step by step how this returned 9 ?

Thank you

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Answer 1

& operator works with binary representation of a number:

25 is 00011001 15 is 00001111

00011001
       &
00001111
--------
00001001

Which is 9 in decimal.