Strange behaviour with numbers that have a leading zero

Question

I have some PHP code with some integers and all works fine, except when I have 08 or 0X as integer. It all works fine when I put them in quote.

Example numbers:

2      //Works fine
08     //Doesn't work
012    //Doesn't work
"08"   //Works fine again
"012"  //Works fine again

Can anyone tell me the reason behind it?

0 in front of a number is considered as Octal number by PHP. — web-nomad, Feb 20 '13 at 13:33
Suggest you read about how to get the number of days in a month, and how to use loops, as well as how to use integers — Mark Baker, Feb 20 '13 at 13:37
I concur. Very inefficient code http://stackoverflow.com/questions/1686724/php-last-day-of-the-month — mplungjan, Feb 20 '13 at 13:38

score 13 · Accepted Answer · edited Jun 20 '20 at 09:12

If you simply write 08 and 09 (without quotes) or any other numeric with a leading 0, PHP believes you're writing an octal value, and 08 and 09 are invalid octal numbers.

http://www.php.net/manual/en/language.types.integer.php

Syntax

Integers can be specified in decimal (base 10), hexadecimal (base 16), octal (base 8) or binary (base 2) notation, optionally preceded by a sign (- or +).

Binary integer literals are available since PHP 5.4.0.

To use octal notation, precede the number with a 0 (zero). To use hexadecimal notation precede the number with 0x. To use binary notation precede the number with 0b.

[...]

Warning: Prior to PHP 7, if an invalid digit was given in an octal integer (i.e. 8 or 9), the rest of the number was ignored. Since PHP 7, a parse error is emitted.

Strange behaviour with numbers that have a leading zero

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