致命错误：调用未定义函数random_bytes（）[重复]

I'm using php 5.6.23-0+deb8u1 and in my code I want use the function random_bytes but I get this error:

Fatal error: Call to undefined function random_bytes()

I want to know if I need to import something or isn't included in this version of php, in this case, how can I replace it?

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random_bytes() was introduced with PHP 7.

As stated in the manual:

Note: Although this function was added to PHP in PHP 7.0, a » userland implementation is available for PHP 5.2 to 5.6, inclusive.

You can use that userland implementation as a backport: https://github.com/paragonie/random_compat

random_bytes was introduced with PHP7 (reference).

Alternatively for old PHP versions you can use mt_rand to generate a random value or rand() to generate a random integer.