The index problem occurs ALWAYS that you are looking for an index in the array that does not exist.

Small examples:

In case of an array of numeric indexes:

<?php
$array = array('Ricdun', 'Markferum', 'Phiaan');
$indice = 3;

echo $array[ $indice ];

In this case it will result:

NOTICE Undefined offset: 3 on line number 4

  

Remember that array of this type always starts at zero, so the last element would be 2 .

In case of an array whose index is not numeric, such as:

$array = array('indice' => 'valor', 'nome' => 'Jenord', 'idade' => '12');

// Exibe nome:
echo $array[ 'nome' ]
// Ok! 

// Exibe pais:
echo $array[ 'pais' ]
// Erro!

This is because the index with the name pais does not exist, so the same error is returned.

Application in your case:

In your case, what can happen is this:

$array = array(
0 => 'Maria',
1 => 'Ana',
3 => 'João',
4 => 'Pedro',
7 => 'Lucas',
8 => 'Vitor',
9 => 'Luisa',
11 => 'Gustavo'
);

for($i = 0; $i <  count($array); $i++){
echo $array[$i];
echo '<br>';
}

Note that the index does not keep an exact order, for example, there is no index of number 2, 5, 6 and nor number 10, but I will get there.

  

Note: The lack of order in the index promotes two problems!

Maria
Ana

NOTICE Undefined offset: 2 on line number 13

João
Pedro

NOTICE Undefined offset: 5 on line number 13


NOTICE Undefined offset: 6 on line number 13

Lucas

Note that the PHP itself points out missing indexes. But note that it will not show the result of " Vitor ", " Luisa " and " Gustavo " and does not even point to error in index 10 .

This is because there are actually only 8 elements, but the last index is 11, higher than the number that will be obtained in count() . This way the loop is closed when $i < 8 , so it will end in 7 , stopping at " Lucas ".

Correction:

There are two easy ways to fix this.

1. Foreach :

Replace for() with foreach .

$array = array(0 => 'Maria', 1 => 'Ana', 3 => 'João', 4 => 'Pedro', 7 => 'Lucas', 8 => 'Vitor', 9 => 'Luisa', 11 => 'Gustavo');

foreach($array as $indice => $valor){

  echo '| Valor: '.$valor;
  echo '<br>';
  echo '| Indice: '.$indice;
  echo '<br><br>';

}

Result:

| Valor: Maria
| Indice: 0

| Valor: Ana
| Indice: 1

| Valor: João
| Indice: 3

| Valor: Pedro
| Indice: 4

| Valor: Lucas
| Indice: 7

| Valor: Vitor
| Indice: 8

| Valor: Luisa
| Indice: 9

| Valor: Gustavo
| Indice: 11

The difference, as you can see, is that foreach automatically ignores nonexistent indices. For foreach it does not matter which index is used.

Documentation: link

  

This method is recommended because indice stays the same!

2. array_values () + [loop] :

If the index does not matter and you do not need it, you can use array_values , so it will rewrite the array in a new array, having the correct order.

$array = array(0 => 'Maria', 1 => 'Ana', 3 => 'João', 4 => 'Pedro', 7 => 'Lucas', 8 => 'Vitor', 9 => 'Luisa', 11 => 'Gustavo');

$array = array_values ( $array );

for($indice = 0; $indice <  count($array); $indice++){

  echo '| Valor: '.$array[ $indice ];
  echo '<br>';
  echo '| Indice: '.$indice;
  echo '<br><br>';

}

Resulting:

| Valor: Maria
| Indice: 0

| Valor: Ana
| Indice: 1

| Valor: João
| Indice: 2

| Valor: Pedro
| Indice: 3

| Valor: Lucas
| Indice: 4

| Valor: Vitor
| Indice: 5

| Valor: Luisa
| Indice: 6

| Valor: Gustavo
| Indice: 7

Check that in this case the Indice is changed, so it keeps the order from 0 to 7. Therefore the old condition of $i < 8 fits correctly.