UTF-8 is capable of encoding all 1,112,064 valid Unicode code points using one to four one-byte (8-bit) code units.

The maximum valid code point in Unicode is U+10FFFF, which makes it a 21-bit code set (but not all 21-bit integers are valid Unicode code points; specifically the values from 0x110000 to 0x1FFFFF are not valid Unicode code points).

mb_chr outputs nothing if you input any number greater than 0x10FFFF.

0x10FFFF-9:1114102=􏿶(4 bytes)
0x10FFFF-8:1114103=􏿷(4 bytes)
0x10FFFF-7:1114104=􏿸(4 bytes)
0x10FFFF-6:1114105=􏿹(4 bytes)
0x10FFFF-5:1114106=􏿺(4 bytes)
0x10FFFF-4:1114107=􏿻(4 bytes)
0x10FFFF-3:1114108=􏿼(4 bytes)
0x10FFFF-2:1114109=􏿽(4 bytes)
0x10FFFF-1:1114110=􏿾(4 bytes)
0x10FFFF+0:1114111=􏿿(4 bytes)
0x10FFFF+1:1114112=(0 bytes)
0x10FFFF+2:1114113=(0 bytes)
0x10FFFF+3:1114114=(0 bytes)
0x10FFFF+4:1114115=(0 bytes)
0x10FFFF+5:1114116=(0 bytes)
0x10FFFF+6:1114117=(0 bytes)
0x10FFFF+7:1114118=(0 bytes)
0x10FFFF+8:1114119=(0 bytes)
0x10FFFF+9:1114120=(0 bytes)