Description:
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If you pass to base_convert a numeric string like "0.5", to have it converted 
from base 10 to base 16, PHP will interpret the input as decimal 5 and return 
hexadecimal 5.  I suspect that the problem lies internally in the logic of 
_php_math_basetozval.

Test script:
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var_dump(base_convert("0.5",10,16));

Expected result:
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I expected PHP to return a zero just as printf("%x","0.5") and dexhex() do.

Actual result:
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5