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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:

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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Subject: PHP

Q:

I am trying to assign a variable the value of 0123, but it keeps coming up with a different number, what’s the problem?

Answer PHP Interpreter treats numbers beginning with 0 as octal. Look at the similar PHP interview questions for more numeric problems.
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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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Q:

What’s the difference between accessing a class method via -> and via ::?

Answer :: is allowed to access methods that can perform static operations, i.e. those, which do not require object initialization.
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Q:

When are you supposed to use endif to end the conditional statement?

Answer When the original if was followed by : and then the code block without braces.
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Subject: PHP