Put your values in an array and then implode
that array with a comma (+ a space to be cleaner):
$myArray = array();
foreach ($this->sinonimo as $s){
$myArray[] = '<span>'.ucfirst($s->sinonimo).'</span>';
}
echo implode( ', ', $myArray );
This will put commas inbetween each value, but not at the end. Also in this case the comma will be outside the span, like:
<span>Text1<span>, <span>Text2<span>, <span>Text3<span>
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