fix deprecated warning php 7.4

htdocs/contact/list.php

@@ -76,7 +76,7 @@ $search_no_email = GETPOST("search_no_email", 'int');
 if (!empty($conf->socialnetworks->enabled)) {
 	foreach ($socialnetworks as $key => $value) {
 		if ($value['active']) {
-			$search_{$key} = GETPOST("search_".$key, 'alpha');
+			${"search_".$key} = GETPOST("search_".$key, 'alpha');
 		}
 	}
 }
@@ -259,7 +259,7 @@ if (empty($reshook))
 		if (!empty($conf->socialnetworks->enabled)) {
 			foreach ($socialnetworks as $key => $value) {
 				if ($value['active']) {
-					$search_{$key} = "";
+					${"search_".$key} = "";
 				}
 			}
 		}
@@ -363,8 +363,8 @@ if (strlen($search_phone_mobile))   $sql .= natural_search('p.phone_mobile', $se
 if (strlen($search_fax))            $sql .= natural_search('p.fax', $search_fax);
 if (!empty($conf->socialnetworks->enabled)) {
 	foreach ($socialnetworks as $key => $value) {
-		if ($value['active'] && strlen($search_{$key})) {
-			$sql .= ' AND p.socialnetworks LIKE \'%"'.$key.'":"'.$search_{$key}.'%\'';
+		if ($value['active'] && strlen(${"search_".$key})) {
+			$sql .= ' AND p.socialnetworks LIKE \'%"'.$key.'":"'.${"search_".$key}.'%\'';
 		}
 	}
 }
@@ -674,7 +674,7 @@ if (!empty($conf->socialnetworks->enabled)) {
 			if (!empty($arrayfields['p.'.$key]['checked']))
 			{
 				print '<td class="liste_titre">';
-				print '<input class="flat" type="text" name="search_'.$key.'" size="6" value="'.dol_escape_htmltag($search_{$key}).'">';
+				print '<input class="flat" type="text" name="search_'.$key.'" size="6" value="'.dol_escape_htmltag(${"search_".$key}).'">';
 				print '</td>';
 			}
 		}