Fallback.php

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<?php
class Fallback {

    public static function mb_substr( $str, $start, $count = 'end' ) {
        if ( $start != 0 ) {
            $split = self::mb_substr_split_unicode( $str, intval( $start ) );
            $str = substr( $str, $split );
        }

        if ( $count !== 'end' ) {
            $split = self::mb_substr_split_unicode( $str, intval( $count ) );
            $str = substr( $str, 0, $split );
        }

        return $str;
    }

    public static function mb_substr_split_unicode( $str, $splitPos ) {
        if ( $splitPos == 0 ) {
            return 0;
        }

        $byteLen = strlen( $str );

        if ( $splitPos > 0 ) {
            if ( $splitPos > 256 ) {
                // Optimize large string offsets by skipping ahead N bytes.
                // This will cut out most of our slow time on Latin-based text,
                // and 1/2 to 1/3 on East European and Asian scripts.
                $bytePos = $splitPos;
                while ( $bytePos < $byteLen && $str[$bytePos] >= "\x80" && $str[$bytePos] < "\xc0" ) {
                    ++$bytePos;
                }
                $charPos = mb_strlen( substr( $str, 0, $bytePos ) );
            } else {
                $charPos = 0;
                $bytePos = 0;
            }

            while ( $charPos++ < $splitPos ) {
                ++$bytePos;
                // Move past any tail bytes
                while ( $bytePos < $byteLen && $str[$bytePos] >= "\x80" && $str[$bytePos] < "\xc0" ) {
                    ++$bytePos;
                }
            }
        } else {
            $splitPosX = $splitPos + 1;
            $charPos = 0; // relative to end of string; we don't care about the actual char position here
            $bytePos = $byteLen;
            while ( $bytePos > 0 && $charPos-- >= $splitPosX ) {
                --$bytePos;
                // Move past any tail bytes
                while ( $bytePos > 0 && $str[$bytePos] >= "\x80" && $str[$bytePos] < "\xc0" ) {
                    --$bytePos;
                }
            }
        }

        return $bytePos;
    }

    public static function mb_strlen( $str, $enc = '' ) {
        $counts = count_chars( $str );
        $total = 0;

        // Count ASCII bytes
        for ( $i = 0; $i < 0x80; $i++ ) {
            $total += $counts[$i];
        }

        // Count multibyte sequence heads
        for ( $i = 0xc0; $i < 0xff; $i++ ) {
            $total += $counts[$i];
        }
        return $total;
    }

    public static function mb_strpos( $haystack, $needle, $offset = 0, $encoding = '' ) {
        $needle = preg_quote( $needle, '/' );

        $ar = array();
        preg_match( '/' . $needle . '/u', $haystack, $ar, PREG_OFFSET_CAPTURE, $offset );

        if ( isset( $ar[0][1] ) ) {
            return $ar[0][1];
        } else {
            return false;
        }
    }

    public static function mb_strrpos( $haystack, $needle, $offset = 0, $encoding = '' ) {
        $needle = preg_quote( $needle, '/' );

        $ar = array();
        preg_match_all( '/' . $needle . '/u', $haystack, $ar, PREG_OFFSET_CAPTURE, $offset );

        if ( isset( $ar[0] ) && count( $ar[0] ) > 0 &&
            isset( $ar[0][count( $ar[0] ) - 1][1] ) ) {
            return $ar[0][count( $ar[0] ) - 1][1];
        } else {
            return false;
        }
    }
}