**The str_replace() Function**

The `str_replace()` function in PHP is used to replace all occurrences of a specified substring with another substring within a string. It takes three required parameters and an optional fourth parameter.

**Syntax:**

“`php
str_replace(find, replace, subject, [count])
“`

**Parameters:**

* **find:** The substring to be replaced.
* **replace:** The substring to replace with.
* **subject:** The string to be searched and replaced in.
* **count:** (Optional) A variable that will hold the number of replacements made.

**Return Value:**

The `str_replace()` function returns a new string with all occurrences of the `find` substring replaced with the `replace` substring. If the `find` substring is not found in the `subject` string, the original string is returned unchanged.

**Example:**

The following code replaces all occurrences of the substring “old” with the substring “new” in the string “This is an old string.”:

“`php
$new_string = str_replace(“old”, “new”, “This is an old string.”);
echo $new_string; // Output: This is an new string.
“`

**Usage:**

The `str_replace()` function can be used to perform a variety of string manipulation tasks, including:

* Replacing placeholders with values in templates.
* Removing unwanted characters from strings.
* Converting strings to different formats.

**Advantages:**

* The `str_replace()` function is simple to use and understand.
* It is efficient and can handle large strings without performance issues.
* It is supported in all versions of PHP.

**Disadvantages:**

* The `str_replace()` function can be inefficient if the `find` substring is found multiple times in the `subject` string.
* It does not support regular expressions.

**Alternatives:**

* The `preg_replace()` function can be used to perform more complex string replacements using regular expressions.
* The `substr_replace()` function can be used to replace a substring at a specific position in a string.