Как использовать пагинацию PHP в массиве

Для доступа к методам массива (json_decode(), count()) вам сначала необходимо array_slice() ваш JSON. Кроме того, ваши данные JSON были недействительными, отсутствовали некоторые запятые, это исправлено.

<?php

$persons = '[ 
{"FrstName":"Henry","Middlename":"","LastName":"Walton","Online":true,"DeptId":"4"},
{"FrstName":"Klaus","Middlename":"","LastName":"Mikaelson","Online":true,"DeptId":"2"},
{"FrstName":"Kylo","Middlename":"","LastName":"Ren","Online":false,"DeptId":"4"},
{"FrstName":"Stan","Middlename":"","LastName":"Lee","Online":false,"DeptId":"3"},
{"FrstName":"Kevin","Middlename":"","LastName":"McNally","Online":false,"DeptId":"3"},
{"FrstName":"Katherine","Middlename":"","LastName":"Pierce","Online":false,"DeptId":"2"},
{"FrstName":"Clint","Middlename":"","LastName":"Barton","Online":true,"DeptId":"3"},
{"FrstName":"Avery","Middlename":"","LastName":"Walton","Online":true,"DeptId":"4"},
{"FrstName":"Peter","Middlename":"","LastName":"Kap","Online":true,"DeptId":"2"},
{"FrstName":"Denzo","Middlename":"","LastName":"Falc","Online":false,"DeptId":"4"},
{"FrstName":"Eveline","Middlename":"","LastName":"Benzel","Online":false,"DeptId":"3"},
{"FrstName":"Bill","Middlename":"","LastName":"Libuz","Online":false,"DeptId":"3"},
{"FrstName":"April","Middlename":"","LastName":"Gonzo","Online":false,"DeptId":"2"},
{"FrstName":"Harry","Middlename":"","LastName":"Geraldson","Online":true,"DeptId":"3"},
{"FrstName":"Heraldson","Middlename":"","LastName":"McGree","Online":false,"DeptId":"3"},
{"FrstName":"Abraham","Middlename":"","LastName":"Retz","Online":true,"DeptId":"4"},
{"FrstName":"June","Middlename":"","LastName":"Pharee","Online":true,"DeptId":"2"},
{"FrstName":"Anthony","Middlename":"","LastName":"Gonzales","Online":false,"DeptId":"4"},
{"FrstName":"Billy","Middlename":"","LastName":"Scott","Online":false,"DeptId":"3"},
{"FrstName":"Anika","Middlename":"","LastName":"Rose","Online":false,"DeptId":"3"},
{"FrstName":"Kristen","Middlename":"","LastName":"Fontana","Online":false,"DeptId":"2"},
{"FrstName":"Olivia","Middlename":"","LastName":"Menzel","Online":true,"DeptId":"3"},
{"FrstName":"Mark","Middlename":"van","LastName":"Gad","Online":false,"DeptId":"3"},
{"FrstName":"Hope","Middlename":"van","LastName":"Dyne","Online":false,"DeptId":"3"}
]';

// Make the JSON an array, so count() and array_slice() work
$persons = json_decode($persons, true);

$page = ! empty( $_GET['page'] ) ? (int) $_GET['page'] : 1;
$total = count( $persons ); //total items in array    
// $limit = 10; //per page
// Set limit to 3 for testing:
$limit = 3;
$totalPages = ceil( $total/ $limit ); //calculate total pages
$page = max($page, 1); //get 1 page when $_GET['page'] <= 0
$page = min($page, $totalPages); //get last page when $_GET['page'] > $totalPages
// Uncomment this for testing
// $page = 2;
$offset = ($page - 1) * $limit;
if ( $offset < 0 ) $offset = 0;

$yourDataArray = array_slice( $persons, $offset, $limit );


$link = 'index.php?page=%d';
$pagerContainer = '<div style = "width: 300px;">'; 
if ( $totalPages != 0 ) 
{
  if ( $page == 1 ) 
  { 
    $pagerContainer .= ''; 
  } 
  else 
  { 
    $pagerContainer .= sprintf( '<a href = "' . $link . '" style = "color: #c00"> &#171; prev page</a>', $page - 1 ); 
  }
  $pagerContainer .= ' <span> page <strong>' . $page . '</strong> from ' . $totalPages . '</span>'; 
  if ( $page == $totalPages ) 
  { 
    $pagerContainer .= ''; 
  }
  else 
  { 
    $pagerContainer .= sprintf( '<a href = "' . $link . '" style = "color: #c00"> next page &#187; </a>', $page + 1 ); 
  }           
}                   
$pagerContainer .= '</div>';

echo $pagerContainer;

foreach($yourDataArray as $person) {
    echo "\n";
    echo "First name: " . $person['FrstName'];
    echo " - Middle name: " . $person['Middlename'];
    echo " - Last name: " . $person['LastName'];
    echo " - Online: " . $person['Online'];
    echo " - Dept: " . $person['DeptId'];
}

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