but other people see them too so it does not look good

on my homepage I get:

Notice [PHP]: Only variables should be assigned by reference in file include/common.php line 223
Notice [PHP]: Only variables should be assigned by reference in file kernel/object.php line 819
Notice [PHP]: Only variables should be assigned by reference in file kernel/object.php line 819
Notice [PHP]: Only variables should be assigned by reference in file kernel/object.php line 819
Notice [PHP]: Only variables should be assigned by reference in file kernel/blockinstance.php line 169
Notice [PHP]: Only variables should be assigned by reference in file kernel/block.php line 179
Notice [PHP]: Only variables should be assigned by reference in file kernel/object.php line 350
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 321
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 112
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 340
Notice [PHP]: Only variables should be assigned by reference in file kernel/block.php line 228
Notice [PHP]: Only variables should be assigned by reference in file kernel/object.php line 350
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 321
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 340
Notice [PHP]: Only variables should be assigned by reference in file kernel/block.php line 228
Notice [PHP]: Only variables should be assigned by reference in file kernel/object.php line 350
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 321
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 119
Notice [PHP]: Only variables should be assigned by reference in file class/module.textsanitizer.php line 340
Notice [PHP]: Only variables should be assigned by reference in file kernel/block.php line 228


but i dont know what it means