How does Casting work in PHP?

What doesn't this work:

(int)08 == (int)09==0

But this and开发者_JS百科 this does?

(int)07==7 
(int)06==6 

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08 is in octal base (because it starts with a 0), hence it is invalid. See the documentation.

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because 08 and 09 are not valid octal numbers. see warning in docs.

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You're type casting an invalid number in octal base.

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// Syntax error
//(int)08 == (int)09==0

// This works
(int)08==0;
(int)09==0;

// This also works
(int)08 == ((int)09==0);

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you are explicitly typecasting using (int)

Better use intval().

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To use hexadecimal notation precede the number with 0x.

Therefore,

 $num = (int)0x9
 $num == 9