corepure64 is the 64-bit version of tinycore, you just need to install it.
If you already have the 32-bit version installed, it's a matter of copying the distribution files corepure64.gz and vmlinuz64 to the same place as core.gz and vmlinuz, adding a new boot entry to your boot loader and adding the boot code "tce=tce64" or similar to keep the extensions separate.
Note that, as mentioned, you can send a pm to the 32-bit php5 maintainer and request an update.