The function behaves differently depending on whether the property has been present in the class declaration, or has been added dynamically, if the variable has been unset()

<?php

class TestClass {

    public $declared = null;
    
}

$testObject = new TestClass;

var_dump(property_exists("TestClass", "dynamic")); // boolean false, as expected
var_dump(property_exists($testObject, "dynamic")); // boolean false, same as above

$testObject->dynamic = null;
var_dump(property_exists($testObject, "dynamic")); // boolean true

unset($testObject->dynamic);
var_dump(property_exists($testObject, "dynamic")); // boolean false, again.

var_dump(property_exists($testObject, "declared")); // boolean true, as espected

unset($testObject->declared);
var_dump(property_exists($testObject, "declared")); // boolean true, even if has been unset()

If you are in a namespaced file, and you want to pass the class name as a string, you will have to include the full namespace for the class name - even from inside the same namespace:

<?
namespace MyNS;

class A {
    public $foo;
}

property_exists("A", "foo");          // false
property_exists("\\MyNS\\A", "foo");  // true
?>

<?php

class Student {

    protected $_name;
    protected $_email;
    

    public function __call($name, $arguments) {
        $action = substr($name, 0, 3);
        switch ($action) {
            case 'get':
                $property = '_' . strtolower(substr($name, 3));
                if(property_exists($this,$property)){
                    return $this->{$property};
                }else{
                    echo "Undefined Property";
                }
                break;
            case 'set':
                $property = '_' . strtolower(substr($name, 3));
                if(property_exists($this,$property)){
                    $this->{$property} = $arguments[0];
                }else{
                    echo "Undefined Property";
                }
                
                break;
            default :
                return FALSE;
        }
    }

}

$s = new Student();
$s->setName('Nanhe Kumar');
$s->setEmail('[email protected]');
echo $s->getName(); //Nanhe Kumar
echo $s->getEmail(); // [email protected]
$s->setAge(10); //Undefined Property
?>

As of PHP 5.3.0, calling property_exists from a parent class sees private properties in sub-classes.

<?php
class P {
    public function test_prop($prop) { return property_exists($this, $prop); }
}

class Child extends P {
    private $prop1;
}

$child = new Child();
var_dump($child->test_prop('prop1')); //true, as of PHP 5.3.0

$a = array('a','b'=>'c');
print_r((object) $a);
var_dump( property_exists((object) $a,'0'));
var_dump( property_exists((object) $a,'b'));

OUTPUT:
stdClass Object
(
    [0] => a
    [b] => c
)
bool(false)
bool(true)

I needed a method for finding out if accessing a property outside a class is possible without errors/warnings, considering that the class might use the magic methods __isset/__get to simulate nonexistent properties.

<?php
// returns true if property is safely accessible publicly by using $obj->$prop
// Tested with PHP 5.1 - 8.2, see https://3v4l.org/QBTd1
function public_property_exists( $obj, $prop ){
  // allow magic $obj->__isset( $prop ) to execute if exists
  if( isset( $obj->$prop ))  return true;
  
  // no public/protected/private property exists with this name
  if( ! property_exists( $obj, $prop ))  return false;

  // the property exists, but is it public?
  $rp = new ReflectionProperty( $obj, $prop );
  return $rp->isPublic();
}

//// Test/demo
class C {
  public    $public    = "I’m public!";
  protected $protected = "I’m public!";
  private   $private   = "I’m public!";
  function __isset( $k ){
    return substr( $k, 0, 5 ) === 'magic';
  }
  function __get( $k ){
    if( $k === 'magic_isset_but_null')  return null;
    return "I’m {$k}!";
  }
}

$o = new C();
foreach( array(
  'public', 'protected', 'private',
  'magic', 'magic_isset_but_null',
  'missing'
) as $prop ){
  if( public_property_exists( $o, $prop ))
        echo "\$o->{$prop} is a public property, its value is: ",
             var_export( $o->$prop, true ), "\n";
  else  echo "\$o->{$prop} is not a public property.\n";
}
/*
$o->public    is a public property, its value is: 'I’m public!'
$o->protected is not a public property.
$o->private   is not a public property.
$o->magic     is a public property, its value is: 'I’m magic!'
$o->magic_isset_but_null is a public property, its value is: NULL
$o->missing   is not a public property.
*/