Question

Reffer this link.

I know operand with string type translate to number, and then usual math

But see these sample codes:

echo intval(1e1);       // 10
var_dump("1e1" == 10);  // true, and it's ok

echo intval(0x1A);      // 26
var_dump("0x1A" == 26); // true, and it's ok

echo intval(042);       // 34
var_dump("042" == 34);  // fasle, Why ?!!!

Why last code return false.

Answer 1

That’s because string-to-number conversion in PHP is based on some ancient C function – strtod. And its rules are as follows:

The expected form of the (initial portion of the) string is optional
leading white space as recognized by isspace(3), an optional plus
(‘+’) or minus sign (‘-‘) and then either (i) a decimal number, or
(ii) a hexadecimal number […]

A decimal number consists of a nonempty sequence of decimal digits
possibly containing a radix character (decimal point,
locale-dependent, usually ‘.’), optionally followed by a decimal
exponent. A decimal exponent consists of an ‘E’ or ‘e’, followed by
an optional plus or minus sign, followed by a nonempty sequence of
decimal digits, and indicates multiplication by a power of 10. [… ]

A hexadecimal number consists of a “0x” or “0X” followed by a nonempty
sequence of hexadecimal digits possibly containing a radix character,
optionally followed by a binary exponent. […]

As you see, ‘1e1’ string has non-empty sequence ‘1’ followed by a decimal exponent ‘e1’. So, it will be converted into a decimal number – and becomes 10.

‘0x1A’ string follows the rules for hexadecimal number, and will be converted into 26 accordingly. But as there’s no specific rule for octadecimal number, ‘042’ will be converted into a plain decimal – and becomes 42. Which is, of course, not equal to 34.

This should not be confused with how number literals are parsed by PHP itself. A number literal that starts with 0 is considered representing an octadecimal. So, intval(042) is essentially the same as intval(34) – but not the same as intval("042").