character encoding conversion UTF-8; transformers converters (by php) of letters in different format like single eight-bit code à © to letter "é"

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Looks like you originally had a 
UTF-8 file 
which has been interpreted as an 8 bit encoding (e.g. ISO-8859-15) 
and entity-encoded. 

ISO-8859-15
http://en.wikipedia.org/wiki/ISO/IEC_8859-15
ISO 8859-15 encodes what it refers to as "Latin alphabet no. 9". This character set is used throughout the Americas, Western Europe, Oceania, and much of Africa.
Each character is encoded as a single eight-bit code value. 

See below for a list of encoding systems.

I say this because the sequence C3A9 looks like a pretty plausible UTF-8 encoding sequence.
http://en.wikipedia.org/wiki/UTF-8#Description
The W3C recommends UTF-8 as the default encoding in XML and HTML.

You will need to first entity-decode it,
then you'll have a UTF-8 encoding again.

iconv

You could then use something like iconv to convert to an encoding of your choosing.
iconv
http://www.gnu.org/savannah-checkouts/gnu/libiconv/documentation/libiconv-1.13/iconv.1.html
The iconv program converts text from one encoding to another encoding.
https://en.wikipedia.org/wiki/Iconv

iconv : unix, mac OSX, linux
https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man1/iconv.1.html

iconv-lite-js

Convert character encodings in pure javascript:
https://github.com/ashtuchkin/iconv-lite

example

To work through your example:

• Ã ©     would be decoded as the byte sequence 0xC3A9
• 0xC3A9 =  11000011     10101001      in binary
                     octetFist       octetSecond
• the leading 110 in the first octet tells us this could be interpreted as a UTF-8 two byte sequence. 
• second octet starts with  10, we're looking at something we can interpret as UTF-8. 
• To do that, we take the last 5 bits of the first octet, and the last 6 bits of the second octet...
• So, interpreted as UTF-8 it's
  00011101001
  = E9
  = é (LATIN SMALL LETTER E WITH ACUTE
  http://www.fileformat.info/info/unicode/char/e9/index.htm)

You mention wanting to handle this with PHP, something like this might do it for you:

 //to load from a file, use
 //$file=file_get_contents("/path/to/filename.txt");
 //example below uses a literal string to demonstrate technique...

 $file="&Précédent is a French word";
 $utf8=html_entity_decode($file);
 $iso8859=utf8_decode($utf8);

 //$utf8 contains "Précédent is a French word" in UTF-8
 //$iso8859 contains "Précédent is a French word" in ISO-8859

Ref.
http://stackoverflow.com/questions/4177783/xc3-xa9-and-other-codes

run OSX Mac (php is installed)

1/ create this text file (a php script: decode/recode)
<?php
 echo "decode-recode*";
 $file="&Pr&#xC3;&#xA9;c&#xC3;&#xA9;dent is a French word";
 $utf8a=html_entity_decode($file);
 $iso8859a=utf8_decode($utf8a);
 echo $utf8a, "//";
 echo $iso8859a, "//";
?>

2/save as "decode1.hph"

3/use the application "terminal" :

php  /path/YYY/decode1.php


4/you will get:

decode-recode*&Précédent is a French word//&Précédent is a French word//

list of encoding systems

in terminal >>iconv --list gives

ANSI_X3.4-1968 ANSI_X3.4-1986 ASCII CP367 IBM367 ISO-IR-6 ISO646-US ISO_646.IRV:1991 US US-ASCII CSASCII

UTF-8 UTF8

UTF-8-MAC UTF8-MAC

ISO-10646-UCS-2 UCS-2 CSUNICODE

UCS-2BE UNICODE-1-1 UNICODEBIG CSUNICODE11

UCS-2LE UNICODELITTLE

ISO-10646-UCS-4 UCS-4 CSUCS4

UCS-4BE

UCS-4LE

UTF-16

UTF-16BE

UTF-16LE

UTF-32

UTF-32BE

UTF-32LE

UNICODE-1-1-UTF-7 UTF-7 CSUNICODE11UTF7

UCS-2-INTERNAL

UCS-2-SWAPPED

UCS-4-INTERNAL

UCS-4-SWAPPED

C99

JAVA

CP819 IBM819 ISO-8859-1 ISO-IR-100 ISO8859-1 ISO_8859-1 ISO_8859-1:1987 L1 LATIN1 CSISOLATIN1

ISO-8859-2 ISO-IR-101 ISO8859-2 ISO_8859-2 ISO_8859-2:1987 L2 LATIN2 CSISOLATIN2

ISO-8859-3 ISO-IR-109 ISO8859-3 ISO_8859-3 ISO_8859-3:1988 L3 LATIN3 CSISOLATIN3

ISO-8859-4 ISO-IR-110 ISO8859-4 ISO_8859-4 ISO_8859-4:1988 L4 LATIN4 CSISOLATIN4

CYRILLIC ISO-8859-5 ISO-IR-144 ISO8859-5 ISO_8859-5 ISO_8859-5:1988 CSISOLATINCYRILLIC

ARABIC ASMO-708 ECMA-114 ISO-8859-6 ISO-IR-127 ISO8859-6 ISO_8859-6 ISO_8859-6:1987 CSISOLATINARABIC

ECMA-118 ELOT_928 GREEK GREEK8 ISO-8859-7 ISO-IR-126 ISO8859-7 ISO_8859-7 ISO_8859-7:1987 ISO_8859-7:2003 CSISOLATINGREEK

HEBREW ISO-8859-8 ISO-IR-138 ISO8859-8 ISO_8859-8 ISO_8859-8:1988 CSISOLATINHEBREW

ISO-8859-9 ISO-IR-148 ISO8859-9 ISO_8859-9 ISO_8859-9:1989 L5 LATIN5 CSISOLATIN5

ISO-8859-10 ISO-IR-157 ISO8859-10 ISO_8859-10 ISO_8859-10:1992 L6 LATIN6 CSISOLATIN6

ISO-8859-11 ISO8859-11 ISO_8859-11

ISO-8859-13 ISO-IR-179 ISO8859-13 ISO_8859-13 L7 LATIN7

ISO-8859-14 ISO-CELTIC ISO-IR-199 ISO8859-14 ISO_8859-14 ISO_8859-14:1998 L8 LATIN8

ISO-8859-15 ISO-IR-203 ISO8859-15 ISO_8859-15 ISO_8859-15:1998 LATIN-9

ISO-8859-16 ISO-IR-226 ISO8859-16 ISO_8859-16 ISO_8859-16:2001 L10 LATIN10

KOI8-R CSKOI8R

KOI8-U

KOI8-RU

CP1250 MS-EE WINDOWS-1250

CP1251 MS-CYRL WINDOWS-1251

CP1252 MS-ANSI WINDOWS-1252

CP1253 MS-GREEK WINDOWS-1253

CP1254 MS-TURK WINDOWS-1254

CP1255 MS-HEBR WINDOWS-1255

CP1256 MS-ARAB WINDOWS-1256

CP1257 WINBALTRIM WINDOWS-1257

CP1258 WINDOWS-1258

850 CP850 IBM850 CSPC850MULTILINGUAL

862 CP862 IBM862 CSPC862LATINHEBREW

866 CP866 IBM866 CSIBM866

MAC MACINTOSH MACROMAN CSMACINTOSH

MACCENTRALEUROPE

MACICELAND

MACCROATIAN

MACROMANIA

MACCYRILLIC

MACUKRAINE

MACGREEK

MACTURKISH

MACHEBREW

MACARABIC

MACTHAI

HP-ROMAN8 R8 ROMAN8 CSHPROMAN8

NEXTSTEP

ARMSCII-8

GEORGIAN-ACADEMY

GEORGIAN-PS

KOI8-T

CP154 CYRILLIC-ASIAN PT154 PTCP154 CSPTCP154

MULELAO-1

CP1133 IBM-CP1133

ISO-IR-166 TIS-620 TIS620 TIS620-0 TIS620.2529-1 TIS620.2533-0 TIS620.2533-1

CP874 WINDOWS-874

VISCII VISCII1.1-1 CSVISCII

TCVN TCVN-5712 TCVN5712-1 TCVN5712-1:1993

ISO-IR-14 ISO646-JP JIS_C6220-1969-RO JP CSISO14JISC6220RO

JISX0201-1976 JIS_X0201 X0201 CSHALFWIDTHKATAKANA

ISO-IR-87 JIS0208 JIS_C6226-1983 JIS_X0208 JIS_X0208-1983 JIS_X0208-1990 X0208 CSISO87JISX0208

ISO-IR-159 JIS_X0212 JIS_X0212-1990 JIS_X0212.1990-0 X0212 CSISO159JISX02121990

CN GB_1988-80 ISO-IR-57 ISO646-CN CSISO57GB1988

CHINESE GB_2312-80 ISO-IR-58 CSISO58GB231280

CN-GB-ISOIR165 ISO-IR-165

ISO-IR-149 KOREAN KSC_5601 KS_C_5601-1987 KS_C_5601-1989 CSKSC56011987

EUC-JP EUCJP EXTENDED_UNIX_CODE_PACKED_FORMAT_FOR_JAPANESE CSEUCPKDFMTJAPANESE

MS_KANJI SHIFT-JIS SHIFT_JIS SJIS CSSHIFTJIS

CP932

ISO-2022-JP CSISO2022JP

ISO-2022-JP-1

ISO-2022-JP-2 CSISO2022JP2

CN-GB EUC-CN EUCCN GB2312 CSGB2312

GBK

CP936 MS936 WINDOWS-936

GB18030

ISO-2022-CN CSISO2022CN

ISO-2022-CN-EXT

HZ HZ-GB-2312

EUC-TW EUCTW CSEUCTW

BIG-5 BIG-FIVE BIG5 BIGFIVE CN-BIG5 CSBIG5

CP950

BIG5-HKSCS:1999

BIG5-HKSCS:2001

BIG5-HKSCS BIG5-HKSCS:2004 BIG5HKSCS

EUC-KR EUCKR CSEUCKR

CP949 UHC

CP1361 JOHAB

ISO-2022-KR CSISO2022KR

CP856

CP922

CP943

CP1046

CP1124

CP1129

CP1161 IBM-1161 IBM1161 CSIBM1161

CP1162 IBM-1162 IBM1162 CSIBM1162

CP1163 IBM-1163 IBM1163 CSIBM1163

DEC-KANJI

DEC-HANYU

437 CP437 IBM437 CSPC8CODEPAGE437

CP737

CP775 IBM775 CSPC775BALTIC

852 CP852 IBM852 CSPCP852

CP853

855 CP855 IBM855 CSIBM855

857 CP857 IBM857 CSIBM857

CP858

860 CP860 IBM860 CSIBM860

861 CP-IS CP861 IBM861 CSIBM861

863 CP863 IBM863 CSIBM863

CP864 IBM864 CSIBM864

865 CP865 IBM865 CSIBM865

869 CP-GR CP869 IBM869 CSIBM869

CP1125

EUC-JISX0213

SHIFT_JISX0213

ISO-2022-JP-3

BIG5-2003

ISO-IR-230 TDS565

ATARI ATARIST

RISCOS-LATIN1

Example

converts input from the old West-European encoding ISO-8859-9 to Unicode UTF-8

For example, use the application "terminal" :

iconv -f ISO8859-9 -t UTF-8 NameFile

and the inverse
iconv -f UTF-8 -t  ISO8859-9