Description:
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It seems like this is a properly handled situation in 7.1.2RC1, but in older versions, there is no Fatal Error making me question if there may be a problem with older PHP (such as version PHP 5.6.17 (cli) (built: Jan  8 2016 10:27:48)).

Unserializing some crafted data leads to this error:
php -r "unserialize('a:1:{i:0;O:1:\"H\":01{}i:0;O:1:\"a\":01{yi:0;O:1:\"a\":3000000000{}i:');"
PHP Fatal error:  Possible integer overflow in memory allocation (3000000001 * 32 + 32) in Command line code on line 1

I am submitting this as a security bug so that someone with better knowledge of PHP internals can make sure this is safe behavior.

Test script:
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php -r "unserialize('a:1:{i:0;O:1:\"H\":01{}i:0;O:1:\"a\":01{yi:0;O:1:\"a\":3000000000{}i:');"