easy solution

<?php
find_missing(['a', 'b', 'd', 'e', 'f', 'g']); //c is missing

find_missing(['n', 'p']); //o is missing
function find_missing($letter) {
    $l = new thing($letter);
    $a0 = [
    1 => 26,
    2 => 25,
    3 => 24,
    4 => 23,
5 => 22,
    6 => 21,
7 => 20,
    8 => 19,
    9 => 18,
    10 => 17,
    11 => 16,
    26 => 1,
    25 => 2,
    24 => 3,
    23 => 4,
    21 => 6,
    22 => 5,
    20 => 7,
    12 => 15,
    13 => 14,
    14 => 13,
    15 => 12,
    16 => 11,
    17 => 10,
    18 => 9,
    19 => 8
    ];
    $letters = [
        'a',
        'b',
        'c',
        'd',
        'e',
        'f',
        'g',
        'h',
        'i','j','k','l','m','n','o','p',
        'q','r','s','t','u','v','w','x','y','z'
    ];
    $has = array();
    foreach($letters as $i => $m) {
        $ll = $letter;
        $zz = array_search(ord($letters[$i]) - 96, $a0);

        if(method_exists(new thing(null), "has" . chr($zz + 64))) {
            $i = ("has" . chr($zz+64));
            if(!(((new thing($ll))->$i($ll))->is_notFALSE())) {
                $has[] = chr($a0[$zz] + 96);
            }
        } else {
            switch($zz) {
            case 20:
                if(!((new thing($ll))->{"hash"}($letter)->is_notFALSE())) {
                $has[] = chr($a0[$zz] + 96);
            }
            break;
            case 11:

                break;

            }
        }
    }
    $hasS = array();
    $last = array_search(reset($letter), $letters);
    $first = array_search(end($letter), $letters);
    foreach($has as $h) {
        $hasS[ord($h)] = $h;
    }
    for($i = 0; $i <= $last + 96; $i++) {
        unset($hasS[$i]);
    }
    for($i = 255; $i > $first + 96; $i--) {
        unset($hasS[$i]);
    }

    echo reset($hasS);
    return end($hasS);
}

class thing {
    function __construct($a) {
        $this->x = $a;
    }

    function hasA($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'z')
                return new self(true);
        }

        return new self(false);
    }

    function is_notFALSE() {
        return !(!$this->x);
    }

    function hasB($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'y')
                return new self(true);
        }

        return new self(false);
    }

    function hasC($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'x')
                return new self(true);
        }

        return new self(false);
    }

    function hasD($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'w')
                return new self(true);
        }

        return new self(false);
    }

    function hasE($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'v')
                return new self(true);
        }

        return new self(false);
    }

    function hasF($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'u')
                return new self(true);
        }

        return new self(false);
    }

    function hasG($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 't')
                return new self(true);
        }

        return new self(false);
    }

    function hash($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 's')
                return new self(true);
        }

        return new self(false);
    }

    function hasI($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'r')
                return new self(true);
        }

        return new self(false);
    }

    function hasJ($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'q')
                return new self(true);
        }

        return new self(false);
    }

    function hasK($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'p')
                return new self(true);
        }

        return new self(false);
    }

    function hasL($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'o')
                return new self(true);
        }

        return new self(false);
    }

    function hasM($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'n')
                return new self(true);
        }

        return new self(false);
    }

    function hasN($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'm')
                return new self(true);
        }

        return new self(false);
    }

    function hasO($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'l')
                return new self(true);
        }

        return new self(false);
    }

    function hasP($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'k')
                return new self(true);
        }

        return new self(false);
    } function hasQ($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'j')
                return new self(true);  } return new self(false);} function hasR($letter) { foreach($letter as $i => $l) {
            if($letter[$i] === 'i')
                return new self(true);
        }

        return new self(false);
    }
    function hasS($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'h')
                return new self(true);
        }

        return new self(false);
    }

    function hasT($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'g')
                return new self(true);
        }

        return new self(false);
    }
    function doesnot_hasU($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'f')
                return new self(false);
        }

        return new self(true);
    } function hasV($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'e')
                return new self(true);
        }

        return new self(false);
    } function hasW($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'd')
                return new self(true);
        }

        return new self(false);
    } function hasX($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'c')
                return new self(true);
        }

        return new self(false);
    } function hasY($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'b')
                return new self(true);
        }

        return new self(false);
    }
    function hasZ($letter) {
        foreach($letter as $i => $l) {
            if($letter[$i] === 'a')
                return new self(true);
        }

        return new self(false);
    }
}