PHP/isset()

解説 変数が存在するとtrueを返す
sample1 yes
sample1code 
$sample1Array = [2,5,6,7,5];
if(isset($sample1Array[4])){
  echo "yes";
} else {
  echo "no";
}
sample2 bool(true)
bool(false)
sample2code 
$s2Str = "ウバメガシ";
var_dump(isset($s2Str));
echo "br />";
var_dump(isset($s2Num));
sample3
no
sample3 
if(isset($_POST["s2Text"])){
  echo "yes";
}else {
  echo "no";
}