str_replace():

                 The str_replace() function replaces some characters with some other characters in a string.

                 This function replaces all occurrences of the search string in the main string with the replace string. Let's look at the example below.

str_replace (search_for_replace, replace_with, string)

                 The first parameter is what you are searching for replace, the second parameter is a string of replace with, and the third parameter is the string you want searched. All three parameters can use an array as input if desired.

Program
<?php 
$str = "Hello! How are you today?";
echo str_replace("Hello", "Hi", $str); 
?>
Output : Hi! How are you today?

                 In the above example, replace the "Hello" with "Hi" in string $str. The above code will output "Hi! How are you today?".