Replacing Substrings

Replacing Substrings


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.


Use substr_replace(), as in Figure.

Replacing a substring with 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);


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 got text that's too big to display all at once, and you want to display some of the text with a link to the rest. Figure displays the first 25 characters of a message with an ellipsis after it as a link to a page that displays more text.

Displaying long text with an ellipsis

$r = mysql_query("SELECT id,message FROM messages WHERE id = $id") or die();
$ob = mysql_fetch_object($r);
printf('<a href="more-text.php?id=%d">%s</a>',
       $ob->id, substr_replace($ob->message,' ...',25));

The more-text.php page referenced in Figure can use the message ID passed in the query string to retrieve the full message and display it.

See Also

Documentation on substr_replace( ) at

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