Undefined index: la upload imagine

[evident]

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<div class="form-group">
    <label>Imagine</label>
    <input type="file" name="avatar">
  </div>

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$fileName = $_FILES["avatar"]["name"]; 
$fileTmpLoc = $_FILES["avatar"]["tmp_name"];
$fileType = $_FILES["avatar"]["type"]; 
$fileSize = $_FILES["avatar"]["size"];
$fileErrorMsg = $_FILES["avatar"]["error"]; 

Undefined index: avatar in toate 5 liniile

Cum trimiti acel fisier la server?
E vreun buton Submit, sau cu javascript?
Incearca sa adaugi tagul input intr-un form.

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<form action='file.php' method='post'>
  <label>Imagine</label>
  <input type='file' name='avatar'>
</form>

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<form  action="add.php" class="form-horizontal"method="post">
 <div class="form-group">
    <label>Imagine</label>
    <input type="file" name="avatar">
  </div>
 <button class="btn btn-primary" type="submit" id="submit">Adauga anuntul</button>
 </form>

Adauga in <form> tagul "enctype", si spatiu intre atribute.

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<form  action="add.php" class="form-horizontal" method="post" enctype="multipart/form-data">