<?php

$string = "abcdaefaedkqatlmtx";

function test($string){

    $len = strlen($string);
    $arr = [];
    
    for ($i = 1; $i <= $len; $i++) {
        for($j = 1; $j<= $len; $j++) {
        
            //从第0个开始依次截取到最大长度，然后从第一个开始...依次截取
            if ($tmp_str = substr($string,$i,$j)) {
                if (checks($tmp_str) !== false) {
                    $arr[strlen($tmp_str)] = $tmp_str;
                } 
            }
        }
    }
    return $arr[max(array_keys($arr))];
}

//此方法用来验证是否包括重复的字符串
function checks($str){
        
    $arr = str_split($str);
    $arr_len = count($arr);
    $tmp_len = count(array_unique($arr));
    
    if ($arr_len  == $tmp_len) {
        return true;
    }
    return false;

}

print_r(test($string));  //结果：edkqatlm

?>

<?php

$string = "abcdaefaedkqatlmtx";
$len = strlen($string);
$resArr =[];
$tmp = [];
$i = 0;
while ($i < $len){
 
            $char = $string{$i};
            if(!array_key_exists($char, $tmp)){
                $tmp[$char]= $i;
                $i++;
                if($i !== $len) continue;
            }
            //从重复值下个开始
            $i = $tmp[$char]+1;
            if( count($tmp) > count($resArr) ){
                $resArr = $tmp;
            }
            $tmp = [];
}

echo implode(array_keys($resArr));  //结果：edkqatlm

?>