PHP is a language that is case sensitive

PHP & upper / lower case

Why is PHP partially case sensitive?

I can only speculate that this is from very early versions, probably PHP / FI 2.0. The manual specifically states:

Note that PHP / FI function names are case-insensitive.

Most of the user input, such as B. GET and POST parameters were always registered as global variables. Treating this information as not being case-sensitive would probably have led to problems, which is why all variables were probably treated as case sensitive.

As far as I know, these were the only types of identifiers in PHP / FI 2.0. All others were introduced later and appear to mimic the names of functions that are not case sensitive.

Constants that are special were not introduced until PHP 4 (the PHP 3 manual mentions "constants", but these are now called "literals"). For some mysterious reason (possibly no consensus could be found) it was decided to allow constant identifier d that distinguishes between upper and lower case for developer discrimination, while uses case sensitive constants by default, the corresponding C counterparts () By default, case is not considered.