- rtrim: documentation ( source)
- str_repeat: documentation ( source)
<?php
$courses = array('Eng', 'Deu', 'Bio', 'Chemi');
$placeholders = rtrim(str_repeat('?, ', count($courses)), ', ');
$query = "select * from table WHERE class = 'EFG' AND course in ({$placeholders})";
echo $query;
/*
$stmt = $pdo->prepare($query);
$stmt->execute($courses);
*/