How can PHP developers efficiently search for and replace substrings within a string using variables?

When searching for and replacing substrings within a string using variables in PHP, developers can use the str_replace() function. This function takes three parameters: the substring to search for, the substring to replace it with, and the original string. By using variables to store the substrings and the original string, developers can efficiently perform multiple replacements within a single string.

// Variables to store substrings and original string
$search = "old";
$replace = "new";
$string = "Replace old with new and old with new again.";

// Perform the replacements
$newString = str_replace($search, $replace, $string);

// Output the new string
echo $newString;