Ordinal value is Nothing but the ASCII value of a character
And as we know every character takes one byte i.e. 8 bits
and every bit can have either 0 or 1 as the possible value
so every bit can have 2 values thus 8 positions can have power(2,8) = 256 combinations
and every combination resembles 1 character like
00000000 => Null(0)
00010000 => Space(32)
(65 - 91) in Ascii => a-z
(97 - 122) in Ascii => (A-Z)
and (48 - 57) in Ascii => (0 - 9)
other combinations are assigned to other special chacater.
PHP have an inbuilt Function ord('a') which takes character as an argument and returns its ascii value i.e. 65 in this case