Description:
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The current behavior on converting a hexadecimal string with an odd-length into binary data results into a warning and returns false.

As the input is in hexadecimal notation it should be very valid to remove "0" from the left.

This behavior is really annoying as it's not enough the check for valid characters it's required to check the length, too.

Additionally within PHP itself the behavior is inconsistent:
	bin2hex(hex2bin("123"))				= Warning: hex2bin(): Hexadecimal input string must have an even length
	dechex(0x123)					= "123"
	base_convert(base_convert("123", 16, 2), 2, 16)	= "123"
	bin2hex(pack('H*', '123'))			= "1230" (pack() will be part of another bug report)

And on looking into JS and (My)SQL it's working well and can result in an odd-length:
JavaScript:
	parseInt("123", 16).toString(16)		= "123"

MySQL:
	SELECT HEX(UNHEX('123'))			= "0123"
	SELECT HEX(0x123)				= "0123"

I have also found #61660 & #65155 where bin2hex(hex2bin("123")) was resulting into "12" or false without any notice which was surly wrong but simply adding a warning and returning false isn't a good fix in my opinion.

Test script:
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bin2hex(hex2bin("123"))

Expected result:
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"0123"

Actual result:
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Warning: hex2bin(): Hexadecimal input string must have an even length