Added documentation for missing ctype extension
Fixed spelling

Fixed spelling.
Updated references to PHP functions

Updated references to PHP functions using syntax suggested by @WouterJ (`:phpfunction:` role).
feature #3601 Added documentation for missing ctype extension (slavafomin)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3601).

Discussion
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Added documentation for missing ctype extension

Commits
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51cfb23 Updated references to PHP functions
bc439ff Fixed spelling
1d03b0e Added documentation for missing ctype extension