Use PHP's substr_replace() method to insert a string into an existing string.  Refer to substr_replace api.

Description

substr_replace ( mixed $string , mixed $replacement , mixed $start [, mixed $length ] ) : mixed

substr_replace() replaces a copy of string delimited by the start and (optionally) length parameters with the string given in replacement.

Parameters

string

The input string.

replacement

The replacement string.

start

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

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

length

If given and is positive, it represents the length of the portion of string which is to be replaced. If it is negative, it represents the number of characters from the end of string at which to stop replacing. If it is not given, then it will default to strlen( string ); i.e. end the replacing at the end of string. Of course, if length is zero then this function will have the effect of inserting replacement into string at the given start offset.

Use it

We set the fourth parameter length of the substr_replace() method to 0.

$oldStr = "abcdefg";
$strToInsert = "123";
$pos = 3;
// In the third position of $oldStr, insert 123.
$newStr = substr_replace($oldStr, $strToInsert, $pos, 0);
echo $oldStr;   // abcdefg
echo $newStr;   // abc123defg