PHP Cookbook

1.2. Replacing Substrings

Problem

You want to replace a substring with a different string. For example, you want to obscure all but the last four digits of a credit card number before printing it.

Solution

Use substr_replace( ) :

// Everything from position $start to the end of $old_string
// becomes $new_substring
$new_string = substr_replace($old_string,$new_substring,$start);

// $length characters, starting at position $start, become $new_substring
$new_string = substr_replace($old_string,$new_substring,$start,$length);

Discussion

Without the $length argument, substr_replace( ) replaces everything from $start to the end of the string. If $length is specified, only that many characters are replaced:

print substr_replace('My pet is a blue dog.','fish.',12);
print substr_replace('My pet is a blue dog.','green',12,4);
$credit_card = '4111 1111 1111 1111';
print substr_replace($credit_card,'xxxx ',0,strlen($credit_card)-4);
My pet is a fish.
               My pet is a green dog.
               xxxx 1111

If $start is negative, the new substring is placed at $start characters counting from the end of $old_string, not from the beginning:

print substr_replace('My pet is a blue dog.','fish.',-9);
print substr_replace('My pet is a blue dog.','green',-9,4);
My pet is a fish.
               My pet is a green dog.

If $start and $length are 0, the new substring is inserted at the start of $old_string:

print substr_replace('My pet is a blue dog.','Title: ',0,0);
Title: My pet is a blue dog.

The function substr_replace( ) is useful when you’ve ...