... 'select login from users where email = :el limit 1', [':el' => $_POST['email']] ...
<?php
$var = 1;
// mysqli
... select * from tab where col1 = ? && col2 = ? && col3 = ?, [$var, $var, $var] ...
// pdo
... select * from tab where col1 = :a && col2 = :a && col3 = :a, [':a' => $var] ...
<?php
$var = 1;
// mysqli
... select * from tab where col1 = ? && col2 = ? && col3 = ?, [$var, $var, $var] ...
// pdo
... select * from tab where col1 = :a && col2 = :a && col3 = :a, [':a' => $var] ...
<?php
$var = 1;
// mysqli
... select * from tab where col1 = ? && col2 = ? && col3 = ?, [$var, $var, $var] ...
// pdo
... select * from tab where col1 = :a && col2 = :a && col3 = :a, [':a' => $var] ...
... 'select login from users where email = :el limit 1', [':el' => $_POST['email']] ...