PHP - Check if string contains same character

You could split on every character then count the array for unique values.

if(count(array_count_values(str_split('abaaaa'))) == 1) {
    echo 'True';
} else {
    echo 'false';
}

Demo: https://eval.in/760293

count(array_unique(explode('', string)) == 1) ? true : false;

You can use a regular expression with a back-reference:

if (preg_match('/^(.)\1*$/', $string)) {
    echo "Same characters";
}

Or a simple loop:

$same = true;
$firstchar = $string[0];
for ($i = 1; $i < strlen($string); $i++) {
    if ($string[$i] != $firstchar) {
        $same = false;
        break;
    }
}