How does php determine whether a constant has been defined?

How to determine whether a php constant has been defined, suddenly confused, dizzy, specially checked the manual, the record filing summary results are as follows:

(1) determine whether a constant exists

If (defined ('myconstant') {echo MYCONSTANT ;}

(2) determine whether a variable is defined

If (isset ($ myvar) {echo "variable $ myvar .";}

(3) determine whether a function exists

If (function_exists ('imap _ open') {echo "function imag_open";} else {echo "function imag_open does not exist ";}

 

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