Description:
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PHP assumes that a string with a leading zero should be interpreted as octal.

In my experience, this is almost never useful, desirable, or intentional, however, it is a frequent trap for the beginner (or more experienced programmer), especially when parsing user-submitted data.

The only reason for this behaviour seems to be historical, and compatibility with strtol(). When non-programmer humans write numbers with leading zeros, they don't usually mean base 8.

My proposal is:

1. Introduce a new string format, 0o####  (letter o), to explicitly mean "this is an octal number". This is similar to the recent introduction of 0b#### for binary numbers. 

[This part should be simple to do, and would also make the intentional use of octal notation explicit, increasing code-readability]


2. Add an option to raise E_NOTICE any time a number is implicitly interpreted as octal (for example  "$x = 0100").


3. In a few years time, (PHP 7?), it may finally be possible to change the default behaviour.

Test script:
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Here's an illustration of a naive program that has data-dependent bugs that are really hard to track down.

$mass_kg = "0.314"      //user-submitted data, known to begin "0."
$mass_g  = substr ($mass_kg, 2);

Yes, this is a silly thing to write. But it's a nasty trap for the unwary.
The above example does what is expected, and might pass many tests. But if the user subsequently enters $mass_kg = "0.031", this will turn into 25 grams, not 31 grams. As a result, we have created a very subtle and hard to find bug.