<?php 
$str = 'abcdefgh'; 
echo str_replace('abc', '123', $str); // 123defgh  
$str = '123456'; 
$search = array(1, 2, 3, 4, 5, 6); 
$replace = array('a', 'b', 'c', 'd', 'f', 'g'); 
echo str_replace($search, $replace, $str); // abcdefg  
$arr = array('abc','bac','cba'); 
$result = str_replace('b', 'B', $arr, $count); 
print_r($result); // Array ( [0] => aBc [1] => Bac [2] => cBa ) 
echo $count;   // 3 共替换了3次 
?> 

<?php 
$str = 'user_order_list'; 
echo str_replace('_', '/', $str); // user/order/list 
?> 

<?php 
/** 
 * 对字符串执行指定次数替换 
 * @param Mixed $search  查找目标值 
 * @param Mixed $replace 替换值 
 * @param Mixed $subject 执行替换的字符串／数组 
 * @param Int  $limit  允许替换的次数，默认为-1，不限次数 
 * @return Mixed 
 */ 
function str_replace_limit($search, $replace, $subject, $limit=-1){ 
  if(is_array($search)){ 
    foreach($search as $k=>$v){ 
      $search[$k] = '`'. preg_quote($search[$k], '`'). '`'; 
    } 
  }else{ 
    $search = '`'. preg_quote($search, '`'). '`'; 
  } 
  return preg_replace($search, $replace, $subject, $limit); 
} 
?> 

<?php 
$str = 'user_order_list'; 
echo str_replace_limit('_', '/', $str, 1); // user/order_list  
$arr = array('abbc','bbac','cbba'); 
$result = str_replace_limit('b', 'B', $arr, 1); 
print_r($result); // Array ( [0] => aBbc [1] => Bbac [2] => cBba ) 
?>