<?php
$sql = "SELECT champ_1, champ_2, ... , champ_n".
"FROM table_x ";
/* En fonction des conditions (champs cochés ou non par exemple, on ajoute les clauses */
$clauses = array();
if(condition_1 == true)
{
    array_push($clauses,"champ_x = valeur_x ");
}
if(condition_2 == true)
{
    array_push($clauses,"champ_y = valeur_y ");
}
/* ... etc ... */
$nb_clauses = count($clauses);
if($nb_clauses > 0)
{
    $sql .= "WHERE ";
    for($i = 0; $i < $nb_clauses; $i++)
    {
        $sql .= ($i == ($nb_clauses - 1)) ? $clauses[$i]."AND " : $clauses[$i];
    }
}
/* Ajout éventuel de clauses supplémentaires comme ORDER BY par exemple */
?>

$query = "SELECT * FROM matable "
$query .= "WHERE ";

if (monchamp1 != ''){ $query .= "matable.monchamp1 = monchamp1  AND ";}
if (monchamp2 != ''){ $query .= "matable.monchamp2 = monchamp2  AND ";}
if (monchamp3 != ''){ $query .= "matable.monchamp3 = monchamp3  AND ";}
if (monchamp4 != ''){ $query .= "matable.monchamp4 = monchamp4 ";}

SELECT * FROM matable WHERE matable.monchamp2 = monchamp2 AND matable.monchamp3 = monchamp3 AND