int variable with leading zero?

but a leading 0 does indicate octal in many languages, as is the case here.

It does have to do with octal numbers, 042 is interpreted as the octal number 42 which is 4 * 8 + 2 = 34.

Please be aware that the octal interpretation happens when the number literal is parsed while loading the PHP script. It has nothing to do with intval(), which doesn't do anything here because the value is already integer.

Octal interpretation happens only with number literals, not when casting a string to integer:

intval(042)   // == 34
intval('042') // == 42
(int)'042'    // == 42

In PHP a number with leading 0 is read as an octal number.

So 042 is read as an octal number.

The intval() function converts it into decimal number, which is 34.

So the browser output is 34.