- strstr: documentation ( source)
<?php
$strselvaluelist = '4|17';
$i=0;
if(!empty($strselvaluelist)){
while($i < strlen($strselvaluelist)){
if($strselvaluelist[$i] == 4){
if(!strstr(17, $strselvaluelist)){
$strselvaluelist.='|17';
}
}
$i++;
}
}