PHPのエラー『Warning: Cannot Use a Scalar Value as an Array』の解決方法

<?php
$scalar = 123;
echo $scalar[0]; // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 'string';
$scalar[0] = 'a'; // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 123;
array_push($scalar, 'value'); // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 'string';
$array = array_merge($scalar, ['value']); // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 123;
$array = [$scalar => 'value']; // これはエラーにならないが、スカラー値を配列として扱うとエラーになる
?>

<?php
$scalar = 'string';
$array = [$scalar];
echo $array[0][0]; // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 123;
$array = ['key' => $scalar];
echo $array['key'][0]; // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 'string';
$array = [];
$array[] = $scalar;
echo $array[0][0]; // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 123;
$array = [];
$array[$scalar] = 'value';
echo $array[$scalar][0]; // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 'string';
$array = [];
$array['key'] = $scalar;
echo $array['key'][0]; // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 123;
$array = [$scalar];
array_push($array[0], 'value'); // Warning: Cannot use a scalar value as an array
?>

<?php
$scalar = 'string';
$array = [$scalar => 'value'];
array_push($array[$scalar], 'value'); // Warning: Cannot use a scalar value as an array
?>