framework/helpers/BaseStringHelper.php
public static function startsWith($string, $with, $caseSensitive = true)
    {
        if (!$bytes = static::byteLength($with)) {
            return true;
        }
        if ($caseSensitive) {
            return strncmp($string, $with, $bytes) === 0;

        }
        $encoding = Yii::$app ? Yii::$app->charset : 'UTF-8';
        return mb_strtolower(mb_substr($string, 0, $bytes, '8bit'), $encoding) === mb_strtolower($with, $encoding);
    }
public static function startsWith($string, $with, $caseSensitive = true)
    {
        if (!$bytes = static::byteLength($with)) {
            return true;
        }
        if ($caseSensitive) {
            return strncmp($string, $with, $bytes) === 0;

        }
        $encoding = Yii::$app ? Yii::$app->charset : 'UTF-8';
        return mb_strtolower(mb_substr($string, 0, $bytes, '8bit'), $encoding) === mb_strtolower($with, $encoding);
    }
Check if given string starts with specified substring.
Binary and multibyte safe.

@param string $string Input string
@param string $with Part to search inside the $string
@param bool $caseSensitive Case sensitive search. Default is true. When case sensitive is enabled, $with must exactly match the starting of the string in order to get a true value.
@return bool Returns true if first input starts with second input, false otherwise