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▼参考配列
$records = array(
    [0] => array(
        'id' => 2135,
        'first_name' => 'John',
    ),
    [1] => array(
        'id' => 3245,
        'first_name' => 'Sally',
    ),
    [3] => array(
        'id' => 5342,
        'first_name' => 'John',
    )
);

▼isset 判定
isset($records)
true

▼array_values 数値添字の値の配列を返す
0,1,2と順に添え字
array_values($records);
Array(
    [0] => array(
        'id' => 2135,
        'first_name' => 'John',
    ),
    [1] => array(
        'id' => 3245,
        'first_name' => 'Sally',
    ),
    [2] => array(
        'id' => 5342,
        'first_name' => 'John',
    )
);

▼in_array 配列に値があるかどうか（1次元）

▼array_keys key検索はこれ（1次元）

▼ array_search + array_column 多階層の時のkey検索はこれ
array_column($records, 'id')
Array
(
    [0] => 2135
    [1] => 3245
    [2] => 5342
)

$id = 2135;
$key = array_search($id, array_column($records, 'id'));
$key = 0;