이름그대로 문자열(string)을 바꾸는(replace)함수인데...
사용법은....

str_replace(바꿀문자,바뀔문자,찾을문자열);

간단한 예)
<?
$word="a";
$word=str_replace("a","b",$word);
echo $word;
?>

<?
$word="12345";
$word=str_replace("1","일",$word);
echo $word;
?>

<?
$word="12345";
$word=str_replace("1","<img src=1.gif>",$word);
echo $word;
?>

<?
$word="12345";
$word=str_replace("0","<img src=0.gif>",$word);
$word=str_replace("1","<img src=1.gif>",$word);
$word=str_replace("2","<img src=2.gif>",$word);
$word=str_replace("3","<img src=3.gif>",$word);
$word=str_replace("4","<img src=4.gif>",$word);
$word=str_replace("5","<img src=5.gif>",$word);
$word=str_replace("6","<img src=6.gif>",$word);
$word=str_replace("7","<img src=7.gif>",$word);
$word=str_replace("8","<img src=8.gif>",$word);
$word=str_replace("9","<img src=9.gif>",$word);
echo $word;
?>


for 응용 예)
<?
$word="12345";

for($i=0;$i<=9;$i++)
{
$word=str_replace("$i","<img src=${i}.gif>",$word);
}

echo $word;
?>