php - TRUE or true difference?

lowercase is generally considered a good practice, see the coloration of the lowercase on stackoverflow :)

And not related, but you should always use true first in the condition true === $var, this is a good practice to avoid sneaky bugs when mistyping the condition, eg : $var = true

There's no difference at all. From the docs:

To specify a boolean literal, use the keywords TRUE or FALSE. Both are case-insensitive.

According to PSR-2, as stated by both Laxus and Paul Bain in their comments, the standard is to write them in lower case.