Description:
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From manual page: http://www.php.net/function.random-bytes
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Examining the C source code, appears that the Error exception might be thrown also on failed reading from the source of randomness, or no enough entropy is available (whenever an estimation of the available entropy is available).

It might worth to mention also that if not enough entropy data is available, some implementations (notably when reading from /dev/urandom as a last resort) may return values from a PRNG rather than from a CSPRNG, so partially defeating the purpose of this function for CS applications.

The same holds for the companion random_int() function, which relies on the same base function as random_bytes().

Programmers should be advised that abusing of the entropy source may then compromise security, and those entropy sources should be used sparingly.