PHP Replace Text Once

replace_text_once.php

<?php
function str_replace_once( $str_pattern, $str_replacement, $string ){ 

    if( strpos( $string, $str_pattern ) !== false )
    { 
        $occurrence = strpos( $string, $str_pattern ); 
        return substr_replace( $string, $str_replacement, strpos( $string, $str_pattern ), strlen( $str_pattern ) ); 
    }
    return $string;
} 

// How to use it:
$text = 'amazon amazon ebay ebay buy at amazon buy at amazon';

$replace_words = array( // text to search => text to replace
    'amazon' => 'Amazon',
    'ebay' => 'Ebay',
    'buy at amazon' => 'buy at amazon'
);
foreach( $replace_words as $key => $value ) {
     $text = str_replace_once( $key, $value, $text );
}
echo $text;
?>