Define undefined array key in php

I’m using the following web to insert two codes separated by the letter “k” in just one field.

<?php
    if(isset($_POST['submitButton'])){
        $input=$_POST['input'];
        $explodedinput=explode('k', $input);

        echo $explodedinput[0];
        echo ' ';
        echo $explodedinput[1];
    }
?>

<form id="login" method="post">
    <input type="password" name="input">
    <input type="submit" name="submitButton" value="SEND">
</form>

The problem comes with the user is wrong and introduces a code without “k” by mistake. Then, it appears the code Warning: Undefined array key 1.

How can I define that value when “k” is not present in the input??

Answer

For PHP 7 and below use strpos or stripos. I created a function like this

function itContains($myString, $search, $caseSensitive = false) {
    return $caseSensitive ?
    (strpos($myString, $search) === FALSE ? FALSE : TRUE):
    (stripos($myString, $search) === FALSE ? FALSE : TRUE);
}

so you may use it like

if(isset($_POST['submitButton'])){
    $input=$_POST['input'];
    if (itContains($input, 'k')) {
      $explodedinput=explode('k', $input);
      echo $explodedinput[0]." ".$explodedinput[1];
    }
   
}

For PHP 8+

if (str_contains('How are you', 'are')) { 
    echo 'yes it contains';
}