A=STRTRAN('KHALID','ALI','123 ')
MESSAGEBOX(A)

Two solutions:
(1) (won't always work) Change it to include spaces pre- and post-:

A=STRTRAN(STRTRAN('KHALID', SPACE(1) + 'ALI','123 '), 'ALI' + SPACE(1), '123)
MESSAGEBOX(A)

(2) (should always work), though you'll have to manually handle punctuation and unusual spacing:

FUNCTION substituteWord
LPARAMETERS tcText, tcWordToSeek, tcWordNew, tlWholeWordOnly

IF tlWholeWordOnly
    * Doing a word at a time
    * Note:  If you are having punctuation or variable spacing, you'll have to handle that manually.
    lcResult = SPACE(0)
    FOR lnI = 1 to GETWORDCOUNT(tcText)
    	lcWord = GETWORDNUM(tcText, lnI)
    	IF lcWord == tcWordToSeek
	    	lcWord = STRTRAN(GETWORDNUM(tcText, lnI), tcWordToSeek, tcWordNew)
	    ENDIF
    	lcResult = lcResult + IIF(EMPTY(lcResult), SPACE(0), SPACE(1)) + lcWord
    NEXT
    * When we get here, every word was replaced
    RETURN lcResult

ELSE
	* Doing any occurrence within
	RETURN STRTRAN(tcText, tcWordToSeek, tcWordNew)
ENDIF

Usage:

? substituteWord("kalid ali kha", "ali", "123", .f.)  && Substitute NOT whole words
? substituteWord("kalid ali kha", "ali", "123", .t.)  && Substitute ONLY whole words