How to Check Array Keys in PHP with array_key_exists and isset

PHP provides the built-in array_key_exists() function to determine whether a specific key name is present in an array. The function accepts two parameters—the key to check and the array to inspect—and returns true if the key exists, otherwise false.

Usage Example:

<?php
// Create an associative array
$student = array(
    "name"   => "张三",
    "age"    => 20,
    "gender" => "男"
);

// Check if the key "name" exists
if (array_key_exists("name", $student)) {
    echo "学生姓名存在于数组中";
} else {
    echo "学生姓名不存在于数组中";
}

// Check if the key "grade" exists
if (array_key_exists("grade", $student)) {
    echo "学生成绩存在于数组中";
} else {
    echo "学生成绩不存在于数组中";
}
?>

Running the script outputs:

学生姓名存在于数组中
学生成绩不存在于数组中

The result shows that the key name is present in $student, while the key grade is not.

Besides array_key_exists(), the isset() function can also test for key existence. The main difference appears when the key's value is null: array_key_exists() returns true, whereas isset() returns false.

Comparison of isset() and array_key_exists():

<?php
// Create an associative array with a null value
$student = array(
    "name"   => "张三",
    "age"    => null,
    "gender" => "男"
);

// Using isset()
if (isset($student["name"])) {
    echo "学生姓名存在于数组中";
} else {
    echo "学生姓名不存在于数组中";
}

if (isset($student["age"])) {
    echo "学生年龄存在于数组中";
} else {
    echo "学生年龄不存在于数组中";
}

// Using array_key_exists()
if (array_key_exists("name", $student)) {
    echo "学生姓名存在于数组中";
} else {
    echo "学生姓名不存在于数组中";
}

if (array_key_exists("age", $student)) {
    echo "学生年龄存在于数组中";
} else {
    echo "学生年龄不存在于数组中";
}
?>

Output of the above code:

学生姓名存在于数组中
学生年龄不存在于数组中
学生姓名存在于数组中
学生年龄存在于数组中

The output demonstrates that isset() treats a null value as non‑existent (returning false), while array_key_exists() treats the same null value as a valid key (returning true).

Overall, array_key_exists() is a reliable tool for checking key presence in PHP arrays, especially when distinguishing between missing keys and keys that hold null values.