PHP looping through an array to fetch a value for each key from database (third normal form)

In addition to your tables

cadets(id, ...)
uniforms(id, ...)

use a cross-product table that describes the relation between entities of cadets and entities of uniforms

cadets_x_uniforms(cadet_id, uniform_id)

For each relationship (in this case cadet x has uniform y) put a record with the respective ids into the cross-product table.

... hm, a working example will do better in this case I suppose...

<?php
$pdo = new PDO('mysql:host=localhost;dbname=test;charset=utf8', 'localonly', 'localonly');
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
setup($pdo);

// fetch the uniforms of a specific cadet
$stmt = $pdo->prepare('
    SELECT
        c.name,u.id,u.labelid
    FROM
        so_cadets as c
    LEFT JOIN
        so_cadet_uniform as cxu
    ON
        c.id=cxu.cadet_id
    LEFT JOIN
        so_uniforms as u
    ON
        cxu.uniform_id=u.id
    WHERE
        c.name=?
');
$stmt->execute( array('cadetB') );
$stmt->setFetchMode(PDO::FETCH_ASSOC);
echo "uniforms of cadetB\n";
foreach( $stmt as $row ){
    echo join(', ', $row), "\n";
}

// fetch cadets without uniforms
$query = '
    SELECT
        c.name
    FROM
        so_cadets as c
    WHERE
        NOT EXISTS(SELECT uniform_id FROM so_cadet_uniform as cxu WHERE c.id=cxu.cadet_id)
';
echo "cadets without uniforms\n";
foreach( $pdo->query($query, PDO::FETCH_ASSOC) as $row ){
    echo join(', ', $row), "\n";
}   


function setup($pdo) {
    $pdo->exec('
        CREATE TEMPORARY TABLE so_cadets (
            id int auto_increment,
            name varchar(32),
            primary key(id)
        )
    ');

    $pdo->exec('
        CREATE TEMPORARY TABLE so_uniforms (
            id int auto_increment,
            labelid varchar(32),
            primary key(id),
            unique key(labelid)
        )
    ');

    $pdo->exec('
        CREATE TEMPORARY TABLE so_cadet_uniform (
            cadet_id int,
            uniform_id int,
            primary key(cadet_id,uniform_id)
        )
    ');


    $stmt = $pdo->prepare('INSERT INTO so_cadets (name) VALUES (?)');
    foreach( range('A','F') as $c ) {
        $stmt->execute( array('cadet'.$c));
    }

    $stmt = $pdo->prepare('INSERT INTO so_uniforms (labelid) VALUES (?)');
    foreach( range('K','Z') as $c ) {
        $stmt->execute( array('label'.$c));
    }

    $stmt = $pdo->prepare('INSERT INTO so_cadet_uniform (cadet_id,uniform_id) VALUES (?,?)');
    $cadetHasUniforms = array(
        1=>array(1,2,3), // <- cadetA
        2=>array(7,9), // <- cadetB
        3=>array(8,5,4), // <- cadetC
        // 4=>array() <- cadetD, no entry, no uniforms
        5=>array(10,13,15) // <- cadetE
        // <- cadetE, no entry, no uniforms
    );
    foreach( $cadetHasUniforms as $cadetId=>$uniformIds ) {
        foreach( $uniformIds as $uid ) {
            $stmt->execute(array($cadetId, $uid));
        }
    }

}

prints

uniforms of cadetB
cadetB, 7, labelQ
cadetB, 9, labelS
cadets without uniforms
cadetD
cadetF