php_falsey_isset_empty.php

<?php
$a = array();

$vals = [
  ['""                        ', ""],
  ['0 (int)                   ', 0],
  ['0.0 (float)               ', 0.0],
  ['"0" (string)              ', "0"],
  ['FALSE                     ', false],
  ['NULL                      ', null],
  ['array()                   ', []],
];

echo "\nisset()\n";

foreach ($vals as list($desc, $value)) {
  echo "\t", $desc, "  ", isset($value) ? "TRUE" : "FALSE", "\n";
}
echo "\t", '$var (declared, no value) ', "  ", isset($var) ? "TRUE" : "FALSE", "\n";
echo "\t", '$a["x"] (undefined offset)', "  ", isset($a['x']) ? "TRUE" : "FALSE", "\n";

echo "\nempty()\n";

foreach ($vals as list($desc, $value)) {
  echo "\t", $desc, "  ", empty($value) ? "TRUE" : "FALSE", "\n";
}
echo "\t", '$var (declared, no value) ', "  ", empty($var) ? "TRUE" : "FALSE", "\n";
echo "\t", '$a["x"] (undefined offset)', "  ", empty($a['x']) ? "TRUE" : "FALSE", "\n";

echo "\nfalseyness\n";

foreach ($vals as list($desc, $value)) {
  echo "\t", $desc, "  ", $value ? "TRUE" : "FALSE", "\n";
}
echo "\t", '$var (declared, no value) ', "  ", @($var ? "TRUE" : "FALSE"), " (PHP Notice)\n";
echo "\t", '$a["x"] (undefined offset)', "  ", @($a['x'] ? "TRUE" : "FALSE"), " (PHP Notice)\n";

/*
 * isset()
 *         ""                          TRUE
 *         0 (int)                     TRUE
 *         0.0 (float)                 TRUE
 *         "0" (string)                TRUE
 *         FALSE                       TRUE
 *         NULL                        FALSE
 *         array()                     TRUE
 *         $var (declared, no value)   FALSE
 *         $a["x"] (undefined offset)  FALSE
 *
 * empty()
 *         ""                          TRUE
 *         0 (int)                     TRUE
 *         0.0 (float)                 TRUE
 *         "0" (string)                TRUE
 *         FALSE                       TRUE
 *         NULL                        TRUE
 *         array()                     TRUE
 *         $var (declared, no value)   TRUE
 *         $a["x"] (undefined offset)  TRUE
 *
 * falseyness
 *         ""                          FALSE
 *         0 (int)                     FALSE
 *         0.0 (float)                 FALSE
 *         "0" (string)                FALSE
 *         FALSE                       FALSE
 *         NULL                        FALSE
 *         array()                     FALSE
 *         $var (declared, no value)   FALSE (PHP Notice)
 *         $a["x"] (undefined offset)  FALSE (PHP Notice)
 */