create function PremierJourApres(@UneDate datetime, @QuelJour integer) returns datetime
as
begin
  return DateAdd(day, 6-((DatePart(dw, @UneDate)+6-@QuelJour) % 7), @UneDate)
end
create function NumeroSemaine(@UneDate datetime) returns integer
as
begin
  declare @PremierJeudi datetime, @DebutSemaine1 datetime
  declare @TheDate datetime, @StartOfYear datetime
  set @TheDate = convert(datetime, convert(varchar, @UneDate, 103), 103)
  set @StartOfYear = DateAdd(day, 1-DatePart(dy, @TheDate), @TheDate)
  set @PremierJeudi = dbo.PremierJourApres(@StartOfYear, 4)
  set @DebutSemaine1 = DateAdd(day, -4, @PremierJeudi)
  return 1+(DateDiff(day, @DebutSemaine1, @UneDate) / 7)
end

function PremierJourApres(UneDate : TDateTime; QuelJour: Integer): TDateTime;
begin
  Result := IncDay(UneDate, 6-((DayOfTheWeek(UneDate)+6-QuelJour) mod 7));
end;

function NumeroSemaine(UneDate : TDateTime): Integer;
var
  PremierJeudi, DebutSemaine1: TDatetime;
begin
  UneDate := DateOf(UneDate);
  PremierJeudi := PremierJourApres(StartOfTheYear(UneDate), 4);
  DebutSemaine1 := IncDay(PremierJeudi, -4);
  Result := 1+(DaysBetween(DebutSemaine1, UneDate) div 7);
end;

//____________________________________________________________________
//
// Retourne le numero de semaine [1 ... 53], 0 si erreur
//____________________________________________________________________
//
int Semaine(int j, int m, int a)
{
  int s = 0;
  struct tm tmj = {0, 0, 12, 1, 0, a - 1900};
  struct tm tmq = {0, 0, 12, j, m - 1, a - 1900};
      
  if(mktime(&tmj) != (time_t)-1 && mktime(&tmq) != (time_t)-1)
  { 
    if(!(s = (tmj.tm_wday + tmq.tm_yday + 6) / 7 - (tmj.tm_wday / 5))) 
      s = Semaine(31, 12, a - 1);
    
    if(m == 12)
    {
      int js = (tmj.tm_wday + tmq.tm_yday) % 7;
      if((js == 1 && (j >= 29 && j <= 31)) ||  
         (js == 2 && (j == 30 || j == 31)) ||  
         (js == 3 &&  j == 31)) s = 1;                        
    }
  }  
  return s;
}

<?php
function NumeroSemaine ($sDate) { // date au format jj/mm/aaaa ou jj-mm-aaaa par exemple
         $sEngDate = substr ($sDate, -4).substr ($sDate, 3, 2).substr ($sDate, 0, 2);
         $iTime = strtotime ($sEngDate);
         return date ('W', $iTime);      
}
function PremierJourApres ($sDate, $iDay) { // iDay : 1 = lundi, 7 = dimanche - sDate = une date format jj/mm/aaaa par exemple
         $sEngDate = substr ($sDate, -4).substr ($sDate, 3, 2).substr ($sDate, 0, 2);
         $iTime = strtotime ($sEngDate);
         $iCur = date ('N', $iTime);
         $iNext = 6 - (($iCur + 6 - $iDay) % 7);
         $iNewTime = $iTime + ($iNext * 86400);
         return strftime ('%d/%m/%Y', $iNewTime);
}
$sDate = '26/05/2006';
echo NumeroSemaine ($sDate);
echo '<br />';
echo PremierJourApres ($sDate, 4);
?>

Numero est un entier = NuméroDeSemaine(Date)

uses DateUtils;
function WeekOf(const AValue : TDateTime):Word;
function WeekOfTheYear(const AValue : TDateTime):Word;
function WeekOfTheYear(const AValue : TDateTime; var AYear : Word ):Word;

Function WeekNumber(Optional ByVal vDate As Variant) As Byte
    If IsMissing(vDate) Then vDate = Date
    
        Dim iNbJour     As Integer
        Dim iWeekDay    As Integer
        Dim bValTemp    As Byte
        Dim a           As String
        Dim b()         As String
    
    iWeekDay = Weekday(CDate("01/01/" & DatePart("yyyy", vDate)))
    
    Select Case iWeekDay
        Case 1: bValTemp = 5: Case 2: bValTemp = 6: Case 3: bValTemp = 0: Case 4: bValTemp = 1: _
        Case 5: bValTemp = 2: Case 6: bValTemp = 3: Case 7: bValTemp = 4
    End Select
    
    iNbJour = CLng(DateDiff("d", CDate("31/12/" & DatePart("yyyy", vDate) - 1), vDate))
    a = IIf((iNbJour + bValTemp) / 7 < 1, 53, CStr((iNbJour + bValTemp) / 7))
    If VarType(a) = vbString Then b() = Split(a, ","): WeekNumber = b(0): Erase b Else WeekNumber = a
End Function

Sub Exemple_Utilisation()
    MsgBox WeekNumber(#1/8/1990#)
    MsgBox WeekNumber()
End Sub