Insert string at specified position

$newstr = substr_replace($oldstr, $str_to_insert, $pos, 0);

http://php.net/substr_replace

In the above snippet, $pos is used in the offset argument of the function.

offset
If offset is non-negative, the replacing will begin at the offset'th offset into string.

If offset is negative, the replacing will begin at the offset'th character from the end of string.

$str = substr($oldstr, 0, $pos) . $str_to_insert . substr($oldstr, $pos);

substr on PHP Manual

Try it, it will work for any number of substrings

<?php
    $string = 'bcadef abcdef';
    $substr = 'a';
    $attachment = '+++';

    //$position = strpos($string, 'a');

    $newstring = str_replace($substr, $substr.$attachment, $string);

    // bca+++def a+++bcdef
?>

Use the stringInsert function rather than the putinplace function. I was using the later function to parse a mysql query. Although the output looked alright, the query resulted in a error which took me a while to track down. The following is my version of the stringInsert function requiring only one parameter.

function stringInsert($str,$insertstr,$pos)
{
    $str = substr($str, 0, $pos) . $insertstr . substr($str, $pos);
    return $str;
}  

This was my simple solution too append text to the next line after it found the keyword.

$oldstring = "This is a test\n#FINDME#\nOther text and data.";

function insert ($string, $keyword, $body) {
   return substr_replace($string, PHP_EOL . $body, strpos($string, $keyword) + strlen($keyword), 0);
}

echo insert($oldstring, "#FINDME#", "Insert this awesome string below findme!!!");

Output:

This is a test
#FINDME#
Insert this awesome string below findme!!!
Other text and data.