PHP code that inserts a string at the specified position in the string

 
//Insert a string
function str_insert($str, $i, $substr) 
{ 
for($j=0; $j<$i; $j++){ 
$startstr .= $str[$j]; 
} 
for ($j=$i; $j<strlen($str); $j++){ 
$laststr .= $str[$j]; 
} 
$str = ($startstr . $substr . $laststr); 
return $str; 
}