Description:
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According to the MIME standard that we're following, this isn't supposed to work when strict mode is enabled.  Equal-signs are strictly reserved for padding characters at the _end_ of a base64 MIME string.

Leading '=' signs are allowed in each block of 4 bytes because the condition that checks for invalid placement of the '=' signs only checks if (i % 4) == 1, 

Whereas it should check if (i % 4) is 0 or 1.

You should be able to change the circumstances for a valid '=' placement, as follows, to conform to the standard:

// to see if bit position 2 is set (for all values of i = n*4+2 and i = n*4+3)
if (i & 2)
    // means that the '=' is in a valid location



I would also suggest *carefully* looking over the block at the end of the base64_decode() function... as it seems that if (ch == base64_pad) after ch is ALWAYS set to '\0' is an impossible scenario.

Test script:
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<?php 
echo base64_decode('=VGhl=ICc9=JyBz=eW1i=b2xz=IGFy=ZW4n=dCBh=bGxv=d2Vk=IHdo=ZXJl=IGkg=cHV0=IHRo=ZW0g=by5P');
?>


Expected result:
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should return FALSE/NULL/Errornous value

Actual result:
--------------
echos:

The '=' symbols aren't allowed where i put them o.O