Understanding str_pad() with leading zeros

I'm trying to build a php function and discovered some weird behavior and I can't even formulate a proper question, so if anyone can explain what is going on, I would appreciate it.

I'm working with a set of n开发者_开发百科umbers with leading zeros, and its important that they be maintained, but users almost never input leading zeros. So I use this:

$x = 123;
$n = 5;
$x = str_pad((int)$x,$n,"0",STR_PAD_LEFT);
echo $x;

and, as desired, this gets me 00123.

The weird stuff happens when I tested for a user inputting a zero before their number

$x = 0123;
$n = 5;
$x = str_pad((int)$x,$n,"0",STR_PAD_LEFT);
echo $x;

This returns 00083. Same thing happens if a user were to input 00123.

That result has me completely bewildered. Thanks in advance for any explanation of what's going on here.

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Integer literals starting with 0 are interpreted base 8. Your second $x has the value 83. See the manual on integers for details.

The intval() function lets you specify the base if you're reading a user string.

If we're talking about literals, in PHP the literal 0 is decimal, while in C and C++ it is octal. It's the little differences that make life fun.