<?php
// Connect to your MySQL database.
$hostname = "localhost";
$username = "your_username";
$password = "your_password";
$database = "database_name";

mysql_connect($hostname, $username, $password);

// The find and replace strings.
$find = "replace this string";
$replace = "with this";

$loop = mysql_query("SELECT
concat('UPDATE ',table_schema,'.',table_name, ' SET ',column_name, '=replace(',column_name,', ''{$find}'', ''{$replace}'');') AS s
FROM
information_schema.columns
WHERE
table_schema = '{$database}'")
or die ('Cant loop through dbfields: ' . mysql_error());

while ($query = mysql_fetch_assoc($loop))
{
    mysql_query($query['s']);
}