Check if all values in array are the same

All values equal the test value:

// note, "count(array_flip($allvalues))" is a tricky but very fast way to count the unique values.
// "end($allvalues)" is a way to get an arbitrary value from an array without needing to know a valid array key. For example, assuming $allvalues[0] exists may not be true.
if (count(array_flip($allvalues)) === 1 && end($allvalues) === 'true') {


}

or just test for the existence of the thing you don’t want:

if (in_array('false', $allvalues, true)) {

}

Prefer the latter method if you’re sure that there’s only 2 possible values that could be in the array, as it’s much more efficient. But if in doubt, a slow program is better than an incorrect program, so use the first method.

If you can’t use the second method, your array is very large, and the contents of the array is likely to have more than 1 value (especially if the 2nd value is likely to occur near the beginning of the array), it may be much faster to do the following:

/**
 * Checks if an array contains at most 1 distinct value.
 * Optionally, restrict what the 1 distinct value is permitted to be via
 * a user supplied testValue.
 *
 * @param array $arr - Array to check
 * @param null $testValue - Optional value to restrict which distinct value the array is permitted to contain.
 * @return bool - false if the array contains more than 1 distinct value, or contains a value other than your supplied testValue.
 * @assert isHomogenous([]) === true
 * @assert isHomogenous([], 2) === true
 * @assert isHomogenous([2]) === true
 * @assert isHomogenous([2, 3]) === false
 * @assert isHomogenous([2, 2]) === true
 * @assert isHomogenous([2, 2], 2) === true
 * @assert isHomogenous([2, 2], 3) === false
 * @assert isHomogenous([2, 3], 3) === false
 * @assert isHomogenous([null, null], null) === true
 */
function isHomogenous(array $arr, $testValue = null) {
    // If they did not pass the 2nd func argument, then we will use an arbitrary value in the $arr (that happens to be the first value).
    // By using func_num_args() to test for this, we can properly support testing for an array filled with nulls, if desired.
    // ie isHomogenous([null, null], null) === true
    $testValue = func_num_args() > 1 ? $testValue : reset($arr);
    foreach ($arr as $val) {
        if ($testValue !== $val) {
            return false;
        }
    }
    return true;
}

Note: Some answers interpret the original question as (1) how to check if all values are the same, while others interpreted it as (2) how to check if all values are the same and make sure that value equals the test value. The solution you choose should be mindful of that detail.

My first 2 solutions answered #2. My isHomogenous() function answers #1, or #2 if you pass it the 2nd arg.