You must put either \; or + at the end of the -exec action.
\;
+
-exec
find . -type f -name '*.php' -print -exec iconv -f euc-kr -t utf-8 {} -o {}.utf8 \;
after some tests finally found out solution working on my mac
for f in *.TXT; do iconv -f CP1250 -t utf-8 $f > $f.txt; done