- strpos: documentation ( source)
<?php
$words = array('systematic', 'سلام','gear','synthesis','mysterious');
$letters=array('s','m', 'e');
foreach ($words as $w) {
//print "lets check word $w<br>";
$n=0;
foreach ($letters as $l) {
if (strpos($w, $l)!==false) $n++;
}
if ($n>=3) print "$w<br>";
}