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Q:

How do you define a constant?

Answer Via define() directive, like define ("MYCONSTANT", 100);
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Q:

What’s the difference between include and require?

Answer It’s how they handle failures. If the file is not found by require(), it will cause a fatal error and halt the execution of the script. If the file is not found by include(), a warning will be issued, but execution will continue.
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Q:

What’s the difference between md5( ), crc32( ) and sha1( ) crypto on PHP?

Answer The major difference is the length of the hash generated. CRC32 is, evidently, 32 bits, while sha1() returns a 128 bit value, and md5() returns a 160 bit value. This is important when avoiding collisions.
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Q:

What is the difference between characters 23 and x23?

Answer The first one is octal 23, the second is hex 23
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Q:

What’s the special meaning of __sleep and __wakeup?

Answer __sleep returns the array of all the variables than need to be saved, while __wakeup retrieves them.
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Subject: PHP

Q:

If the variable $a is equal to 5 and variable $b is equal to character a, what’s the value of $$b?

Answer 100, it’s a reference to existing variable.
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Q:

Will comparison of string "10" and integer 11 work in PHP?

Answer Yes, internally PHP will cast everything to the integer type, so numbers 10 and 11 will be compared.
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Q:

So if md5( ) generates the most secure hash, why would you ever use the less secure crc32( ) and sha1( )?

Answer Crypto usage in PHP is simple, but that doesn’t mean it’s free. First off, depending on the data that you’re encrypting, you might have reasons to store a 32-bit value in the database instead of the 160-bit value to save on space. Second, the more secure the crypto is, the longer is the computation time to deliver the hash value. A high volume site might be significantly slowed down, if frequent md5() generation is required.
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Subject: PHP